doc: Fix description of datatype_isbitsegal (#61379)

I wrote this originally, but it doesn't really make sense as written.
Two issues:

1. `Implies` implies injection, not bijection and moreover the
implication we actually care about is the other way around (i.e. does
semantic egality imply bits-egality, because then we can do the the
cheap bits-compare, rather than calling `===` on every field.
2. The word `pointers` was missing.