Improve comments for IncrementalMaintenance DML enabling functions.

Move the static functions after the comment and expand the comment.

Per complaint from Andres Freund, although using different comment
text.

src/backend/commands/matview.c

@@ -816,27 +816,33 @@ refresh_by_heap_swap(Oid matviewOid, Oid OIDNewHeap)
 	RelationCacheInvalidateEntry(matviewOid);
 }
 
-static void
-OpenMatViewIncrementalMaintenance(void)
-{
-	matview_maintenance_depth++;
-}
-
-static void
-CloseMatViewIncrementalMaintenance(void)
-{
-	matview_maintenance_depth--;
-	Assert(matview_maintenance_depth >= 0);
-}
 
 /*
  * This should be used to test whether the backend is in a context where it is
  * OK to allow DML statements to modify materialized views.  We only want to
  * allow that for internal code driven by the materialized view definition,
  * not for arbitrary user-supplied code.
+ *
+ * While the function names reflect the fact that their main intended use is
+ * incremental maintenance of materialized views (in response to changes to
+ * the data in referenced relations), they are initially used to allow REFRESH
+ * without blocking concurrent reads.
  */
 bool
 MatViewIncrementalMaintenanceIsEnabled(void)
 {
 	return matview_maintenance_depth > 0;
 }
+
+static void
+OpenMatViewIncrementalMaintenance(void)
+{
+	matview_maintenance_depth++;
+}
+
+static void
+CloseMatViewIncrementalMaintenance(void)
+{
+	matview_maintenance_depth--;
+	Assert(matview_maintenance_depth >= 0);
+}