Doc: Improve documentation for creating custom scan paths.

This improves the documentation related to parallel query a little bit.

Reviewed by Richard Guo.

Discussion: https://postgr.es/m/CAPmGK159eJShAR4ek4Db8oHD4%2BZ18zwJkisEWkqRN-80BdDGFQ%40mail.gmail.com

doc/src/sgml/custom-scan.sgml

@@ -38,8 +38,8 @@
     A custom scan provider will typically add paths for a base relation by
     setting the following hook, which is called after the core code has
     generated all the access paths it can for the relation (except for
-    Gather paths, which are made after this call so that they can use
-    partial paths added by the hook):
+    Gather and Gather Merge paths, which are made after this call so that
+    they can use partial paths added by the hook):
 <programlisting>
 typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
                                             RelOptInfo *rel,
@@ -53,9 +53,10 @@ extern PGDLLIMPORT set_rel_pathlist_hook_type set_rel_pathlist_hook;
     Although this hook function can be used to examine, modify, or remove
     paths generated by the core system, a custom scan provider will typically
     confine itself to generating <structname>CustomPath</structname> objects and adding
-    them to <literal>rel</literal> using <function>add_path</function>.  The custom scan
-    provider is responsible for initializing the <structname>CustomPath</structname>
-    object, which is declared like this:
+    them to <literal>rel</literal> using <function>add_path</function>, or
+    <function>add_partial_path</function> if they are partial paths.  The
+    custom scan provider is responsible for initializing the
+    <structname>CustomPath</structname> object, which is declared like this:
 <programlisting>
 typedef struct CustomPath
 {