[clang] [analyzer] Fix crash of StreamChecker when eval calling 'fopen' (PR #100990)

[Balazs Benics via cfe-commits]

steakhal wrote:

> This looks good now, documentation could be a bit more exact in that operations on standard streams are not checked by the checker, like any other operation on streams that are not opened on the analysis path.

Elaborated the docs section.

https://github.com/llvm/llvm-project/pull/100990