RISC-V: Fix minor bugs in .insn docs.

RISC-V: Fix minor bugs in .insn docs.

This fixes some minor bugs in the docs for the .insn directive pointed out
by Frédéric Pétrot, and I added a few more cleanups since I was changing
the docs.

	gas/
	PR 26051
	* doc/c-riscv.texi (RISC-V-Formats): Add missing I format using
	simm12(rs1).  Correct S format to use simm12(rs1).  Drop SB and B
	formats using simm12(rs1).  Correct SB and B to use rs1 and rs2.
	Move B before SB.  Move J before UJ.