[clang] Document -Wglobal-constructors behavior (#68084)

It's a drop in the ocean considering the lack of documentation of our
diagnostics, but it's a start.

Co-authored-by: serge-sans-paille <[email protected]>

Author: serge-sans-paille

clang/include/clang/Basic/DiagnosticGroups.td

@@ -415,7 +415,11 @@ def : DiagGroup<"c++2a-compat-pedantic", [CXX20CompatPedantic]>;
 def ExitTimeDestructors : DiagGroup<"exit-time-destructors">;
 def FlexibleArrayExtensions : DiagGroup<"flexible-array-extensions">;
 def FourByteMultiChar : DiagGroup<"four-char-constants">;
-def GlobalConstructors : DiagGroup<"global-constructors">;
+def GlobalConstructors : DiagGroup<"global-constructors"> {
+ code Documentation = [{
+Emit a warning for each variable declaration that generates code run at startup.
+ }];
+}
 def BitwiseConditionalParentheses: DiagGroup<"bitwise-conditional-parentheses">;
 def BitwiseOpParentheses: DiagGroup<"bitwise-op-parentheses">;
 def LogicalOpParentheses: DiagGroup<"logical-op-parentheses">;