Doc: explicitly point out that enum values can't be dropped.

This was not stated in so many words anywhere.  Document it to make
clear that it's a design limitation and not just an oversight or
documentation omission.

Discussion: https://postgr.es/m/152089733343.1222.6927268289645380498@wrigleys.postgresql.org

Author: tglsfdc

doc/src/sgml/datatype.sgml

@@ -3059,19 +3059,27 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
    <sect2>
     <title>Implementation Details</title>
 
+    <para>
+     Enum labels are case sensitive, so
+     <type>'happy'</type> is not the same as <type>'HAPPY'</type>.
+     White space in the labels is significant too.
+    </para>
+
+    <para>
+     Although enum types are primarily intended for static sets of values,
+     there is support for adding new values to an existing enum type, and for
+     renaming values (see <xref linkend="sql-altertype"/>).  Existing values
+     cannot be removed from an enum type, nor can the sort ordering of such
+     values be changed, short of dropping and re-creating the enum type.
+    </para>
+
     <para>
      An enum value occupies four bytes on disk.  The length of an enum
      value's textual label is limited by the <symbol>NAMEDATALEN</symbol>
      setting compiled into <productname>PostgreSQL</productname>; in standard
      builds this means at most 63 bytes.
     </para>
 
-    <para>
-     Enum labels are case sensitive, so
-     <type>'happy'</type> is not the same as <type>'HAPPY'</type>.
-     White space in the labels is significant too.
-    </para>
-
     <para>
      The translations from internal enum values to textual labels are
      kept in the system catalog