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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Hum, right, the `-std=c94` flag does not exist but maybe we should say instead we support `-std=iso9899:199409` - that does not appear to be documented anywhere

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