Add a comment to absl/flags/parse.cc indicating that the flags defined there are…

Add a comment to absl/flags/parse.cc indicating that the flags defined there are not intended to be read or set from C++ code.

PiperOrigin-RevId: 547264846
Change-Id: Idd0958f308ec74f907c22cce965fac2e06692e2c

absl/flags/parse.cc

@@ -99,6 +99,8 @@ struct SpecifiedFlagsCompare {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+// These flags influence how command line flags are parsed and are only intended
+// to be set on the command line.  Avoid reading or setting them from C++ code.
 ABSL_FLAG(std::vector<std::string>, flagfile, {},
           "comma-separated list of files to load flags from")
     .OnUpdate([]() {
@@ -148,6 +150,8 @@ ABSL_FLAG(std::vector<std::string>, tryfromenv, {},
       absl::flags_internal::tryfromenv_needs_processing = true;
     });
 
+// Rather than reading or setting --undefok from C++ code, please consider using
+// ABSL_RETIRED_FLAG instead.
 ABSL_FLAG(std::vector<std::string>, undefok, {},
           "comma-separated list of flag names that it is okay to specify "
           "on the command line even if the program does not define a flag "