Improve FWU documentation

Improve FWU documentation

 - Clarify the documentation of the 'FWU_SMC_IMAGE_COPY' SMC in the
   Firmware Update guide. Also extend the list of pre-conditions to
   include the additional input validation implemented by previous
   patches.

 - Improve documentation of bl1_plat_mem_check() in the porting
   guide. It now specifies that the generic FWU code protects
   bl1_plat_mem_check() from integer overflows resulting from
   the addition of the base address and size passed in arguments.

Change-Id: I07b47a3778df7b9c089529b2dd2135707640a91c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>