[mips] Explained the 'w' modifier in the Inline Assembler documentation.

[mips] Explained the 'w' modifier in the Inline Assembler documentation.

It exists for compatibility with GCC which requires it to print MSA registers
for the 'f' constraint. Although LLVM doesn't need it, the 'w' modifier should
still be used for portability between the two compilers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242015 91177308-0d34-0410-b5e6-96231b3b80d8