[flang] Document extension

Original-commit: flang-compiler/f18@ca2a7dc5a8
Reviewed-on: https://github.com/flang-compiler/f18/pull/335

flang/documentation/Extensions.md

@@ -58,6 +58,8 @@ Extensions, deletions, and legacy features supported by default
   the length parameter of the implicit type, not the first.
 * Outside a character literal, a comment after a continuation marker (&)
   need not begin with a comment marker (!).
+* Classic C-style /*comments*/ are skipped, so multi-language header
+  files are easier to write and use.
 
 Extensions supported when enabled by options
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