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

[Mingming Liu via llvm-commits]

https://github.com/minglotus-6 commented:

The file name should probably be updated to `InstrumentationPGOProfileFormat` . Will wait and do this later as one-off to minimize diff..

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