[RISCV] Bump Zfa version to 0.2 and correct RISCVUsage description

As of
https://github.com/riscv/riscv-isa-manual/commit/1f038182810727f5feca311072e630d6baac51da
in the riscv-isa-manual, Zfa is at version 0.2. Reviewing the commit
history for
zfa.tex
<https://github.com/riscv/riscv-isa-manual/commits/master/src/zfa.tex>
there are no relevant changes since 0.1. As such, we can simply
increment the version number.

This change also removes the claim in RISCVUsage that we implement a
"subset of" Zfa, as I believe this is no longer true. That sentence
previously incorrectly claimed we didn't implement fli.{h,s,d} (I
[corrected this a couple of weeks
ago](https://reviews.llvm.org/rG3d969191b277)) but I think should have
removed the "subset of" wording too.

As was noted during the review, we never added Zfa to the release notes.
This is corrected in this patch.

Differential Revision: https://reviews.llvm.org/D148634