[clang] [Clang] Document the interaction of malloc_span and alloc_size (PR #216967)

[Aleksandr Nogikh via cfe-commits]

https://github.com/a-nogikh approved this pull request.

I think some rewording would be nice, otherwise it sounds reasonable that the documentation should be more explicit here.

https://github.com/llvm/llvm-project/pull/216967