docs/kernel-parameters: update description of mem=

The existing description of mem= does not cover all the cases and
differences between how architectures treat it.

Extend the description to match the code.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20220310082736.1346366-1-rppt@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>