Export of internal doc changes to Abseil OSS: rpc://team/absl-team/Abseil-Docs
Export of internal doc changes to Abseil OSS:
rpc://team/absl-team/Abseil-Docs

Included changes:

249902651(shreck):	Add flags library document to main C++ guides list
249646985(Abseil Team):	Fix a compile error in the sample code in the flags docs.

PiperOrigin-RevId: 249902651
Change-Id: I0022eb2d570d660260706cc5c27e549a0a197287

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