powerpc/slb: Add documentation on runtime patching of SLB encoding

This patch adds some documentation to patch_slb_encoding() explaining
how it works.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Update change log and mention the signedness of the immediate]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>