docs: Clarify that signal functions have no feedback.

Bug #16652 complains that pg_reload_conf() returned true, even though
the configuration file contained errors. That's the way pg_reload_conf()
works, by design, but the documentation wasn't very clear on it. Clarify
that a 'true' return value only means that the signal was sent
successfully. Also add links to the system views that can be used to
check the configuration files for errors.

David G. Johnston, with some rewording by me.

Discussion: https://www.postgresql.org/message-id/CAKFQuwax6GxhUQEes0D045UtXG-fBraM39_6UMd5JyR5K1HWCQ%40mail.gmail.com

doc/src/sgml/func.sgml

@@ -23922,7 +23922,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
 
    <para>
     Each of these functions returns <literal>true</literal> if
-    successful and <literal>false</literal> otherwise.
+    the signal was successfully sent and <literal>false</literal>
+    if sending the signal failed.
    </para>
 
    <table id="functions-admin-signal-table">
@@ -23970,7 +23971,10 @@ SELECT collation for ('foo' COLLATE "de_DE");
         server to reload their configuration files.  (This is initiated by
         sending a <systemitem>SIGHUP</systemitem> signal to the postmaster
         process, which in turn sends <systemitem>SIGHUP</systemitem> to each
-        of its children.)
+        of its children.) You can use the
+        <link linkend="view-pg-file-settings">pg_file_settings</link> and
+        <link linkend="view-pg-hba-file-rules">pg_hba_file_rules</link> views
+        to check the configuration files for possible errors, before reloading.
        </para></entry>
       </row>