doc: clarify that auth. names are lower case and case-sensitive

This is true even for acronyms that are usually upper case, like LDAP.

Reported-by: Alvaro Herrera

Discussion: https://postgr.es/m/202205141521.2nodjabmsour@alvherre.pgsql

Backpatch-through: 10

doc/src/sgml/client-auth.sgml

@@ -417,7 +417,9 @@ hostnogssenc  <replaceable>database</replaceable>  <replaceable>user</replaceabl
       <para>
        Specifies the authentication method to use when a connection matches
        this record. The possible choices are summarized here; details
-       are in <xref linkend="auth-methods"/>.
+       are in <xref linkend="auth-methods"/>.  All the options
+       are lower case and treated case sensitively, so even acronyms like
+       <literal>ldap</literal> must be specified as lower case.
 
        <variablelist>
         <varlistentry>