Doc: Fix description of pg_class.relreplident

The description missed a comma and lacked an explanation of what happens
with REPLICA IDENTITY USING INDEX when the dependent index is dropped.

Author: Marina Polyakova
Reviewed-by: Daniel Gustafsson, Michael Paquier
Discussion: https://postgr.es/m/ad1a0badc32658b1bbb07aa312346a1d@postgrespro.ru
Backpatch-through: 9.5

doc/src/sgml/catalogs.sgml

@@ -2141,8 +2141,10 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        Columns used to form <quote>replica identity</quote> for rows:
        <literal>d</literal> = default (primary key, if any),
        <literal>n</literal> = nothing,
-       <literal>f</literal> = all columns
-       <literal>i</literal> = index with <structfield>indisreplident</structfield> set, or default
+       <literal>f</literal> = all columns,
+       <literal>i</literal> = index with
+       <structfield>indisreplident</structfield> set (same as nothing if the
+       index used has been dropped)
       </para></entry>
      </row>