[DOC] Remove too strong restriction for ‘llvm.experimental.gc.statepoint’ Intrinsic

The requirement for deopt parameter to be in gc parameter if it can
be modified by GC is very strong and difficult to follow.

The key example of why this can't work:
%p1 = bitcast i8* %p to i8*
statepoint [gc = (%p1)], [deopt = (%p1)]

The optimizer is allowed to replace either use (or both) of %p1 with %p.
If it updates only one of the two (entirely legal), the two sets do not overlap.

So this change removes the strong wording.

Reviewers: reames, dantrushin
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D77122