powerpc/tm: Fix HTM documentation

This patch simply fix part of the documentation on the HTM code.

This fixes reference to old fields that were renamed in commit
000ec280 ("powerpc: tm: Rename transct_(*) to ck(\1)_state")

It also documents better the flow after commit eb5c3f1c ("powerpc:
Always save/restore checkpointed regs during treclaim/trecheckpoint"),
where tm_recheckpoint can recheckpoint what is in ck{fp,vr}_state
blindly.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>