Silence compiler warning on an unused variable

Also, tweak wording in comments (per Andres) and documentation (myself)
to point out that it's the database's default tablespace that can be
passed as 0, not DEFAULTTABLESPACE_OID.  Robert Haas noticed the bug in
the code, but didn't update the accompanying prose.

doc/src/sgml/func.sgml

@@ -15814,8 +15814,8 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    <para>
     <function>pg_filenode_relation</> is the reverse of
     <function>pg_relation_filenode</>. Given a <quote>tablespace</> OID and
-    a <quote>filenode</> it returns the associated relation. The default
-    tablespace can be specified as 0.
+	a <quote>filenode</>, it returns the associated relation's OID. For a table
+    in the database's default tablespace, the tablespace can be specified as 0.
    </para>
 
   </sect2>

src/backend/utils/adt/dbsize.c

@@ -760,13 +760,14 @@ pg_relation_filenode(PG_FUNCTION_ARGS)
  * Get the relation via (reltablespace, relfilenode)
  *
  * This is expected to be used when somebody wants to match an individual file
- * on the filesystem back to its table. Thats not trivially possible via
- * pg_class because that doesn't contain the relfilenodes of shared and nailed
+ * on the filesystem back to its table. That's not trivially possible via
+ * pg_class, because that doesn't contain the relfilenodes of shared and nailed
  * tables.
  *
  * We don't fail but return NULL if we cannot find a mapping.
  *
- * Instead of knowing DEFAULTTABLESPACE_OID you can pass 0.
+ * InvalidOid can be passed instead of the current database's default
+ * tablespace.
  */
 Datum
 pg_filenode_relation(PG_FUNCTION_ARGS)

src/backend/utils/cache/relfilenodemap.c

@@ -210,7 +210,7 @@ RelidByRelfilenode(Oid reltablespace, Oid relfilenode)
 
 	while (HeapTupleIsValid(ntp = systable_getnext(scandesc)))
 	{
-		bool isnull;
+		bool isnull PG_USED_FOR_ASSERTS_ONLY;
 
 		if (found)
 			elog(ERROR,