Merge branch 'jc/document-test-balloons-in-flight'

To help our developers, document what C99 language features are
being considered for adoption, in addition to what past experiments
have already decided.

* jc/document-test-balloons-in-flight:
  CodingGuidelines: document test balloons in flight

Author: gitster

Documentation/CodingGuidelines

@@ -301,6 +301,14 @@ For C programs:
    . since late 2023 with 8277dbe987 we have been using the bool type
      from <stdbool.h>.
 
+   C99 features we have test balloons for:
+
+   . since late 2024 with v2.48.0-rc0~20, we have test balloons for
+     compound literal syntax, e.g., (struct foo){ .member = value };
+     our hope is that no platforms we care about have trouble using
+     them, and officially adopt its wider use in mid 2026.  Do not add
+     more use of the syntax until that happens.
+
    New C99 features that we cannot use yet:
 
    . %z and %zu as a printf() argument for a size_t (the %z being for