Add a note about the scope of the syntax reference

Closes #9067.

Author: José Valim

lib/elixir/pages/Unicode Syntax.md

@@ -34,11 +34,13 @@ Elixir does not allow the use of ZWJ or ZWNJ in identifiers and therefore does n
 
 ### Atoms
 
-Atoms in Elixir follow the identifier rule above with the following modifications:
+Unicode Atoms in Elixir follow the identifier rule above with the following modifications:
 
   * `<Start>` includes the code point `_` (005F)
   * `<Continue>` includes the code point `@` (0040)
 
+> Note that all Elixir operators are also valid atoms. Therefore `:+`, `:@`, `:|>`, and others are all valid atoms. The full description of valid atoms is available in the Syntax Reference, this document covers only the rules for identifier-based atoms.
+
 ### Variables
 
 Variables in Elixir follow the identifier rule above with the following modifications: