powerpc/tm: Document h/rfid and mtmsrd quirk

The ISA has a quirk that's useful for the Linux implementation.
Document it here so others are less likely to trip over it.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200325040546.3091563-1-mikey@neuling.org