Doc: improve documentation of pg_proc.protrftypes.

Add a "references" link pointing to pg_type, as we have for other arrays
of type OIDs.  Wordsmith the explanation a bit.

Joel Jacobson, additional editing by me

Discussion: https://postgr.es/m/d1cc628c-3953-4209-957b-29427acc38c8@www.fastmail.com

doc/src/sgml/catalogs.sgml

@@ -5862,7 +5862,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
-       An array with the data types of the function arguments.  This includes
+       An array of the data types of the function arguments.  This includes
        only input arguments (including <literal>INOUT</literal> and
        <literal>VARIADIC</literal> arguments), and thus represents
        the call signature of the function.
@@ -5875,7 +5875,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
-       An array with the data types of the function arguments.  This includes
+       An array of the data types of the function arguments.  This includes
        all arguments (including <literal>OUT</literal> and
        <literal>INOUT</literal> arguments); however, if all the
        arguments are <literal>IN</literal> arguments, this field will be null.
@@ -5889,7 +5889,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        <structfield>proargmodes</structfield> <type>char[]</type>
       </para>
       <para>
-       An array with the modes of the function arguments, encoded as
+       An array of the modes of the function arguments, encoded as
        <literal>i</literal> for <literal>IN</literal> arguments,
        <literal>o</literal> for <literal>OUT</literal> arguments,
        <literal>b</literal> for <literal>INOUT</literal> arguments,
@@ -5907,7 +5907,7 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
        <structfield>proargnames</structfield> <type>text[]</type>
       </para>
       <para>
-       An array with the names of the function arguments.
+       An array of the names of the function arguments.
        Arguments without a name are set to empty strings in the array.
        If none of the arguments have a name, this field will be null.
        Note that subscripts correspond to positions of
@@ -5932,9 +5932,12 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
        <structfield>protrftypes</structfield> <type>oid[]</type>
+       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
       </para>
       <para>
-       Data type OIDs for which to apply transforms.
+       An array of the argument/result data type(s) for which to apply
+       transforms (from the function's <literal>TRANSFORM</literal>
+       clause).  Null if none.
       </para></entry>
      </row>