[llvm] [compiler-rt] [docs][IRPGO]Document two binary formats for IRPGO profiles (PR #76105)

[Mingming Liu via llvm-commits]

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minglotus-6 wrote:

> Perhaps name this ProfileFormat.rst, since so much is shared with pure code coverage applications.

Ack. I wonder if we want to use `InstrumentationProfileFormat.rst` since SamplePGO uses different format.

https://github.com/llvm/llvm-project/pull/76105