[MSSAU] Clarify that the defining access does not matter (NFC)

New memory accesses are usually inserted by using one of the
createMemoryAccessXYZ() methods followed by insertUse() or
insertDef(). createMemoryAccessXYZ() accepts a defining access,
however this defining access will always be overwritten by
insertUse() / insertDef().

Update the documentation to clarify this, and stop passing
Definition to createMemoryAccessXYZ() if it's followed by
insertUse/insertDef.

Alternatively, we could also make insertUse / insertDef keep the
defining access if it is specified, and only recompute it if it's
missing.

Differential Revision: https://reviews.llvm.org/D157979