[3/15][Clang][RISCV][NFC] Clarify edge cases of...

[3/15][Clang][RISCV][NFC] Clarify edge cases of RVVIntrinsic::getSupportedMaskedPolicies for clarity

This is the 3rd commit of a patch-set that aims to change the default policy
for RVV intrinsics from TAMU to TAMA.

Please refer to the cover letter in the 1st commit (D141573) for an
overview.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D141575