Add note about custom instructions varying behavior (#58511)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: hubwriter

content/copilot/concepts/prompting/response-customization.md

@@ -26,6 +26,8 @@ category:
 
 {% data variables.product.prodname_copilot %} can provide responses that are tailored to your personal preferences, the way your team works, the tools you use, or the specifics of your project, if you provide it with enough context to do so. Instead of repeatedly adding this contextual detail to your prompts, you can create custom instructions that automatically add this information for you. The additional information is not displayed, but is available to {% data variables.product.prodname_copilot_short %} to allow it to generate higher quality responses.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## Types of custom instructions
 
 There are three main types of custom instructions that you can use to customize {% data variables.product.prodname_copilot_short %} responses on the {% data variables.product.github %} website:
@@ -122,6 +124,8 @@ There are two types of files you can use to provide context and instructions to
 
 While custom instructions help to add codebase-wide context to each AI workflow, prompt files let you add instructions to a specific chat interaction.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## About repository custom instructions
 
 You can use three types of repository custom instructions in {% data variables.product.prodname_vscode_shortname %}:
@@ -199,6 +203,8 @@ The following examples demonstrate how to use prompt files.
 
 {% data variables.product.prodname_copilot %} can provide responses that are tailored to the way your team works, the tools you use, or the specifics of your project, if you provide it with enough context to do so. Instead of repeatedly adding this contextual detail to your prompts, you can create a custom instructions file in your repository that automatically adds this information for you. The additional information is not displayed in the chat input box, but is available to {% data variables.product.prodname_copilot_short %} to allow it to generate higher quality responses.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## About repository custom instructions
 
 In {% data variables.product.prodname_vs %}, repository custom instructions consist of a single file, `.github/copilot-instructions.md`, that you create in a repository. The instructions you add to the file should be short, self-contained statements that add context or relevant information to supplement a {% data variables.product.prodname_copilot_short %} prompt.
@@ -246,6 +252,8 @@ There are two types of files you can use to provide context and instructions to
 
 While custom instructions help to add codebase-wide context to each AI workflow, prompt files let you add instructions to a specific chat interaction.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## About repository custom instructions
 
 In JetBrains IDEs, repository custom instructions consist of a single file, `.github/copilot-instructions.md`, that you create in a repository. The instructions you add to the file should be short, self-contained statements that add context or relevant information to supplement a {% data variables.product.prodname_copilot_short %} prompt.
@@ -337,6 +345,8 @@ For a curated collection of examples, see [AUTOTITLE](/copilot/tutorials/customi
 
 {% data variables.product.prodname_copilot %} can provide responses that are tailored to the way your team works, the tools you use, or the specifics of your project, if you provide it with enough context to do so. Instead of repeatedly adding this contextual detail to your prompts, you can create a custom instructions file in your repository that automatically adds this information for you. The additional information is not displayed in the chat input box, but is available to {% data variables.product.prodname_copilot_short %} to allow it to generate higher quality responses.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## About repository custom instructions
 
 In Xcode, repository custom instructions consist of a single file, `.github/copilot-instructions.md`, that you create in a repository. The instructions you add to the file should be short, self-contained statements that add context or relevant information to supplement a {% data variables.product.prodname_copilot_short %} prompt.
@@ -382,6 +392,8 @@ There are two types of repository custom instructions you can use to provide con
 * **Workspace custom instructions** which apply to all projects in a workspace, and allow you to specify workspace-wide instructions and preferences, in a single file.
 * **Project custom instructions** which apply to a specific project.
 
+{% data reusables.copilot.custom-insts-nondeterministic %}
+
 ## About repository custom instructions
 
 **Workspace custom instructions** apply to all projects in a workspace, and allow you to specify workspace-wide instructions and preferences. You can use workspace custom instructions to provide context and instructions to {% data variables.product.prodname_copilot_short %} in Eclipse.

data/reusables/copilot/custom-insts-nondeterministic.md

@@ -0,0 +1,2 @@
+> [!NOTE]
+> Due to the non-deterministic nature of AI, {% data variables.product.prodname_copilot_short %} may not always follow your custom instructions in exactly the same way every time they are used.