psql: Reword help message and docs for WATCH_INTERVAL

Reword the documentation around the default value to make interaction
between WATCH_INTERVAL and the \watch command clearer.  While there,
also remove a stray parenthesis left over from a previous version of
the patch.

Reported-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/c34a650b-6f8b-4da7-9ebb-b6df03ce009d@eisentraut.org

doc/src/sgml/ref/psql-ref.sgml

@@ -3857,7 +3857,7 @@ SELECT 1 \bind \sendpipeline
         (if given) is reached, or the query no longer returns the minimum number
         of rows. Wait the specified number of seconds (default 2) between executions.
         The default wait can be changed with the variable
-        <xref linkend="app-psql-variables-watch-interval"/>).
+        <xref linkend="app-psql-variables-watch-interval"/>.
         For backwards compatibility,
         <replaceable class="parameter">seconds</replaceable> can be specified
         with or without an <literal>interval=</literal> prefix.
@@ -4756,9 +4756,10 @@ bar
         <term><varname>WATCH_INTERVAL</varname></term>
         <listitem>
         <para>
-        This variable sets the default interval which <command>\watch</command>
-        waits between executing the query.  Specifying an interval in the
-        command overrides this variable.
+        This variable sets the default interval, in seconds, which
+        <command>\watch</command> waits between executing the query. The
+        default is 2 seconds.  Specifying an interval in the command overrides
+        this variable.
         </para>
         </listitem>
       </varlistentry>

src/bin/psql/help.c

@@ -463,8 +463,9 @@ helpVariables(unsigned short int pager)
 		  "  VERSION_NAME\n"
 		  "  VERSION_NUM\n"
 		  "    psql's version (in verbose string, short string, or numeric format)\n");
-	HELP0("  WATCH_INTERVAL\n"
-		  "    if set to a number, overrides the default two second \\watch interval\n");
+	HELPN("  WATCH_INTERVAL\n"
+		  "    number of seconds \\watch waits between executions (default %s)\n",
+		  DEFAULT_WATCH_INTERVAL);
 
 	HELP0("\nDisplay settings:\n");
 	HELP0("Usage:\n");