docs: Update documentation about ARMv8.2-TTCNP

Commit 2559b2c8

 ("xlat v2: Dynamically detect need for CnP bit")
modified the code to convert the compile-time check for ARMv8.2-TTCNP to
a runtime check, but forgot to update the documentation associated to it.

Change-Id: I6d33a4de389d976dbdcce65d8fdf138959530669
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>