Docs: describe how key flags work

Currently docs say:
> --help: prints a list of all key flags (see below).

However, this is the only mention of `key` flags. Reading issues I see these are inferred automagically from the main module, and that perhaps `flags.declare_key_flag` can somehow be used to manually declare them, at least in some circumstances. None of this is documented though and scrambling through what little there is, I was not able to make it work in a multi module environment where I want to selectively surface some flags from various modules into the top level `--helpshort`. As an alternative I could centralize flag definitions, but then I would still need documentation on key flags to be able to select from that central definition which are key flags and which are not.

Look at the abseil-py source code for flag definitions and the `declare_key_flag` function. Examine existing documentation files to understand the current structure. Test flag behavior in a simple multi-module setup to verify how key flags are inferred or declared. The goal is to add clear documentation explaining key flags, their automatic inference, and manual declaration with examples.

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