doc: Consistently use restartpoint in the documentation

doc: Consistently use restartpoint in the documentation

The majority of cases already used "restartpoint" with just a few
instances of "restart point". Changing the latter spelling to the
former ensures consistency in the user facing documentation. Code
comments are not affected by this since it is not worth the churn
to change anything there.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/0F6E38D0-649F-4489-B2C1-43CD937E6636@yesql.se