[MSSA][Doc] Clobbers, more info on Defs / Def chain

- Added more info about what we refer as a clobber in MSSA.
- Added more info about MemoryDefs and how there is a single Def chain.
- The doc portrayed MSSA as modeling the heap whileit is modeling
  the whole memory, so I changed the wording to not be heap-specific.

Differential Revision: https://reviews.llvm.org/D80000