doc: Inform about aminsertcleanup optional NULLness

This index AM callback has been introduced in c1ec02be1d and it is
optional, currently only being used by BRIN.  Optional callbacks are
documented with NULL as possible value in amapi.h and indexam.sgml, but
this callback has missed this part of the description.

Reported-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/CAHut+PvgYcPmPDi1YdHMJY5upnyGRpc0N8pk1xNB11xDSBwNog@mail.gmail.com
Backpatch-through: 17

doc/src/sgml/indexam.sgml

@@ -147,7 +147,7 @@ typedef struct IndexAmRoutine
     ambuild_function ambuild;
     ambuildempty_function ambuildempty;
     aminsert_function aminsert;
-    aminsertcleanup_function aminsertcleanup;
+    aminsertcleanup_function aminsertcleanup;   /* can be NULL */
     ambulkdelete_function ambulkdelete;
     amvacuumcleanup_function amvacuumcleanup;
     amcanreturn_function amcanreturn;   /* can be NULL */

src/include/access/amapi.h

@@ -293,7 +293,7 @@ typedef struct IndexAmRoutine
 	ambuild_function ambuild;
 	ambuildempty_function ambuildempty;
 	aminsert_function aminsert;
-	aminsertcleanup_function aminsertcleanup;
+	aminsertcleanup_function aminsertcleanup;	/* can be NULL */
 	ambulkdelete_function ambulkdelete;
 	amvacuumcleanup_function amvacuumcleanup;
 	amcanreturn_function amcanreturn;	/* can be NULL */