[clang] [clang][analyzer doc] Add basic description of checker 'core.CallAndMessage' (NFC). (PR #177179)

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

Lovely. Finally some love to this checker.
Have you generated the html? How would it look?

https://github.com/llvm/llvm-project/pull/177179