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

diff --cc Documentation/CodingGuidelines
index 528b42d1dd1846653fa6307e3f79960d49bad982,0776d15a95be934903f2063e1a7aa361c311326e..f47412042530a9324d0ce7313b8664484dd0dde1
--- 1/Documentation/CodingGuidelines
--- 2/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@@ -298,9 -298,14 +298,17 @@@ For C programs
     . since late 2021 with 44ba10d6, we have had variables declared in
       the for loop "for (int i = 0; i < 10; i++)".
  
 +   . 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