Fix documentation of to_char/to_timestamp TZ, tz, OF formatting patterns.

These are only supported in to_char, not in the other direction, but the
documentation failed to mention that.  Also, describe TZ/tz as printing the
time zone "abbreviation", not "name", because what they print is elsewhere
referred to that way.  Per bug #14558.

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -5988,15 +5988,18 @@ SELECT regexp_matches('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
        </row>
        <row>
         <entry><literal>TZ</literal></entry>
-        <entry>upper case time-zone name</entry>
+        <entry>upper case time-zone abbreviation
+         (only supported in <function>to_char</>)</entry>
        </row>
        <row>
         <entry><literal>tz</literal></entry>
-        <entry>lower case time-zone name</entry>
+        <entry>lower case time-zone abbreviation
+         (only supported in <function>to_char</>)</entry>
        </row>
        <row>
         <entry><literal>OF</literal></entry>
-        <entry>time-zone offset</entry>
+        <entry>time-zone offset from UTC
+         (only supported in <function>to_char</>)</entry>
        </row>
       </tbody>
      </tgroup>