Update docs of memcpy/move/set wrt. align and len

Fix https://bugs.llvm.org/show_bug.cgi?id=38583: Describe
how memcpy/memmove/memset behave when len=0. Also fix
some fallout from when the alignment parameter was
replaced by an attribute.

This closes PR38583.

Patch by RalfJung (Ralf)

Differential Revision: https://reviews.llvm.org/D57600

llvm-svn: 354911