(Freplace_region_contents): Treat point as insert-before marker

(Freplace_region_contents): Treat point as insert-before marker

Experience suggests that it's more often useful to keep point
at the end of the replacement than it is to keep point at the
beginning of the replacement.
This also aligns the behavior of `replace-region-contents` with
that of `insert`.

* src/insdel.c (replace_range): Treat PT like an insert-before marker.
* src/editfns.c (Freplace_region_contents): Adjust docstring accordingly.

(cherry picked from commit 4532fbefece210061d01ab9523f3054aadb1c45a)