doc: Clarify use of _ccnew and _ccold in REINDEX CONCURRENTLY

Invalid indexes are suffixed with "_ccnew" or "_ccold".  The
documentation missed to mention the initial underscore.
ChooseRelationName() may also append an extra number if indexes with a
similar name already exist; let's add a note about that too.

Author: Alec Cozens <[email protected]>
Discussion: https://postgr.es/m/174733277404.1455388.11471370288789479593@wrigleys.postgresql.org
Backpatch-through: 13

Author: michaelpq

doc/src/sgml/ref/reindex.sgml

@@ -465,14 +465,17 @@ Indexes:
 </programlisting>
 
     If the index marked <literal>INVALID</literal> is suffixed
-    <literal>ccnew</literal>, then it corresponds to the transient
+    <literal>_ccnew</literal>, then it corresponds to the transient
     index created during the concurrent operation, and the recommended
     recovery method is to drop it using <literal>DROP INDEX</literal>,
     then attempt <command>REINDEX CONCURRENTLY</command> again.
-    If the invalid index is instead suffixed <literal>ccold</literal>,
+    If the invalid index is instead suffixed <literal>_ccold</literal>,
     it corresponds to the original index which could not be dropped;
     the recommended recovery method is to just drop said index, since the
     rebuild proper has been successful.
+    A nonzero number may be appended to the suffix of the invalid index
+    names to keep them unique, like <literal>_ccnew1</literal>,
+    <literal>_ccold2</literal>, etc.
    </para>
 
    <para>