[clang] [clang][docs]Remove references to c94 standard (PR #206403)

[Hardik Kumar via cfe-commits]

hardikxk wrote:

Yes i had also noticed that clang supports the `-std=iso9899:199409` standard but not c94. I did not add that to the documentation as I wanted a review whether it should be added to the documentation or not.

https://github.com/llvm/llvm-project/pull/206403