[libc-commits] [libc] [libc] Updated header_generation.rst (PR #99712)

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https://github.com/overmighty approved this pull request.

LGTM besides a few more tiny nitpicks. Also, there are trailing spaces in header_generation.rst, but `rg ' +$' {llvm,libc}/docs` shows that it is also the case in existing docs.

https://github.com/llvm/llvm-project/pull/99712