Fix documentation of psql's ECHO all mode.

"ECHO all" is ignored for interactive input, and has been for a very long
time, though possibly not for as long as the documentation has claimed the
opposite.  Fix that, and also note that empty lines aren't echoed, which
while dubious is another longstanding behavior (it's embedded in our
regression test files for one thing).  Per bug #12721 from Hans Ginzel.

In HEAD, also improve the code comments in this area, and suppress an
unnecessary fflush(stdout) when we're not echoing.  That would likely
be safe to back-patch, but I'll not risk it mere hours before a release
wrap.

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

@@ -53,8 +53,8 @@ PostgreSQL documentation
       <term><option>--echo-all</></term>
       <listitem>
       <para>
-      Print all input lines to standard output as they are read. This is more
-      useful for script processing than interactive mode. This is
+      Print all nonempty input lines to standard output as they are read.
+      (This does not apply to lines read interactively.) This is
       equivalent to setting the variable <varname>ECHO</varname> to
       <literal>all</literal>.
       </para>
@@ -2565,13 +2565,13 @@ bar
         <term><varname>ECHO</varname></term>
         <listitem>
         <para>
-        If set to <literal>all</literal>, all lines
-        entered from the keyboard or from a script are written to the standard output
-        before they are parsed or executed. To select this behavior on program
+        If set to <literal>all</literal>, all nonempty input lines are printed
+        to standard output as they are read.  (This does not apply to lines
+        read interactively.)  To select this behavior on program
         start-up, use the switch <option>-a</option>. If set to
         <literal>queries</literal>,
-        <application>psql</application> merely prints all queries as
-        they are sent to the server. The switch for this is
+        <application>psql</application> prints each query to standard output
+        as it is sent to the server. The switch for this is
         <option>-e</option>.
         </para>
         </listitem>