Improve comments for execExpr.c's handling of FieldStore subexpressions.

Improve comments for execExpr.c's handling of FieldStore subexpressions.

Given this code's general eagerness to use subexpressions' output variables
as temporary workspace, it's not exactly clear that it is safe for
FieldStore to tell a newval subexpression that it can write into the same
variable that is being supplied as a potential input.  Document the chain
of assumptions needed for that to be safe.