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

doc/src/sgml/config.sgml

@@ -4050,7 +4050,7 @@ include_dir 'conf.d'
         (The path name is relative to the current working directory,
         i.e., the cluster's data directory.)
         Any <literal>%r</literal> is replaced by the name of the file containing the
-        last valid restart point. That is the earliest file that must be kept
+        last valid restartpoint. That is the earliest file that must be kept
         to allow a restore to be restartable, so this information can be used
         to truncate the archive to just the minimum required to support
         restarting from the current restore. <literal>%r</literal> is typically only
@@ -4095,7 +4095,7 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         cleaning up old archived WAL files that are no longer needed by the
         standby server.
         Any <literal>%r</literal> is replaced by the name of the file containing the
-        last valid restart point.
+        last valid restartpoint.
         That is the earliest file that must be <emphasis>kept</emphasis> to allow a
         restore to be restartable, and so all files earlier than <literal>%r</literal>
         may be safely removed.
@@ -4139,7 +4139,7 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
         <varname>recovery_end_command</varname> is to provide a mechanism for cleanup
         following replication or recovery.
         Any <literal>%r</literal> is replaced by the name of the file containing the
-        last valid restart point, like in <xref linkend="guc-archive-cleanup-command"/>.
+        last valid restartpoint, like in <xref linkend="guc-archive-cleanup-command"/>.
        </para>
        <para>
         If the command returns a nonzero exit status then a warning log