Add section defining when to set misa.C

Add rule that C implies Zca, C+F also implies Zcf (RV32), C+D also implies Zcd

Clarification that Zcf implies F and Zcd implies D
Clarification that Zcf is RV32 only

This release was created by: rpsene

This extension is Ratified. No changes are allowed. Any desired or needed changes can be the subject of a follow-on new extension. Ratified extensions are never revised. See https://riscv.org/spec-state.

Zc* is a group of extensions which define subsets of the existing C extension (Zca, Zcd, Zcf) and new extensions which only contain 16-bit encodings.

Zcm* all reuse the encodings for c.fld, c.fsd, c.fldsp, c.fsdsp.

make register field names consistent for c.mul, c.not, c.zext.b

Updated instruction formats following Architecture Review Committee feedback