Mention vacuum for relpages.

contrib/oid2name/README.oid2name

@@ -115,10 +115,11 @@ $ du -s * |
 2135        18735  = postgres
 ..
 
-This can be done in psql with:  (each page is typically 8k)
+This can be done in psql with:  
 
 test=> SELECT relpages, relfilenode, relname FROM pg_class ORDER BY relpages DESC;
 
+Each page is typically 8k.  Relpages is updated by VACUUM.
 
 ---------------------------------------------------------------------------