[Bug]: Effect of ABSL_FLAGS_STRIP_NAMES on mobile platforms is poorly documented

Look at absl/flags/config.h to see how ABSL_FLAGS_STRIP_NAMES is defined for mobile platforms. Examine absl/flags/flag.h to understand the ABSL_FLAG_IMPL macro and how it registers flags. The documentation update should clarify that setting ABSL_FLAGS_STRIP_NAMES=0 disables flag registration on iOS/Android, and suggest where to add this note (perhaps in a README or a comment in config.h).

Describe the issue

I am porting command line C++ code to use on iOS. Setting aside the issue of actually passing command line arguments to an iOS app (it is possible and fully supported), calling absl::ParseCommandLine(argc, argv) on stack variables seems to fail mysteriously. Even within the same translation unit, ABSL_FLAG definitions are ignored and not present in the .o.

I thought this was an issue with my toolchain configuration (linker stripping, etc). But it turns out, it is due to ABSL_FLAGS_STRIP_NAMES being set to 0 on "mobile" targets, where "mobile" is defined to be Android, iPhone, and "embedded Apple devices".

While explicitly setting ABSL_FLAGS_STRIP_NAMES=0 fixes the issue, the name ABSL_FLAGS_STRIP_NAMES is confusing as its effect is to disable flag registration altogether: ABSL_FLAG_IMPL is defined to call absl::flags_internal::FlagRegistrar<T, /*do_register=*/false> or absl::flags_internal::FlagRegistrar<T, /*do_register=*/true>.

I am not advocating that we change the name - that's extremely disruptive.

But maybe we should better document it somewhere as this behavior is surprising (albeit rare - I don't know how many Abseil users actually want to pass flags to iOS apps. My team for one would like to do this as it conveniently lets us pass things like --stderrthreshold 0.

Steps to reproduce the problem

Write a C++ library that looks like:

ABSL_FLAG(int, value1, 0, "value1");
ABSL_FLAG(double, value2, 0.0, "value2");

void parse() {
  int argc = 4;
  char* argv[4] = {
      "app",
      "--value1=6",
      "--value2",
      "7.6",
  };
  absl::ParseCommandLine(argc, argv);
}

and call parse() from the iOS app. It will mysteriously complain:

ERROR: Unknown command line flag 'value1'
ERROR: Unknown command line flag 'value2'

Building with ABSL_FLAGS_STRIP_NAMES=0 fixes the issue.

What version of Abseil are you using?

20240722.0

What operating system and version are you using?

iOS 18.0

What compiler and version are you using?

Xcode 16.0

$ clang -v
Apple clang version 16.0.0 (clang-1600.0.26.3)
Target: arm64-apple-darwin23.6.0
Thread model: posix
InstalledDir: /Applications/Xcode_16.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

What build system are you using?

bazel 7.1.0

Additional context

No response