Rename CHECKPOINT_IMMEDIATE to CHECKPOINT_FAST.

The new name more accurately reflects the effects of this flag on a
requested checkpoint.  Checkpoint-related log messages (i.e., those
controlled by the log_checkpoints configuration parameter) will now
say "fast" instead of "immediate", too.  Likewise, references to
"immediate" checkpoints in the documentation have been updated to
say "fast".  This is preparatory work for a follow-up commit that
will add a MODE option to the CHECKPOINT command.

Author: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/aDnaKTEf-0dLiEfz%40msg.df7cb.de

doc/src/sgml/backup.sgml

@@ -991,7 +991,7 @@ SELECT pg_backup_start(label => 'label', fast => false);
      usually preferable as it minimizes the impact on the running system.  If you
      want to start the backup as soon as possible, pass <literal>true</literal> as
      the second parameter to <function>pg_backup_start</function> and it will
-     request an immediate checkpoint, which will finish as fast as possible using
+     request a fast checkpoint, which will finish as fast as possible using
      as much I/O as possible.
     </para>
 

doc/src/sgml/func.sgml

@@ -28973,7 +28973,7 @@ LOG:  Grand total: 1651920 bytes in 201 blocks; 622360 free (88 chunks); 1029560
         will be stored.)
         If the optional second parameter is given as <literal>true</literal>,
         it specifies executing <function>pg_backup_start</function> as quickly
-        as possible.  This forces an immediate checkpoint which will cause a
+        as possible.  This forces a fast checkpoint which will cause a
         spike in I/O operations, slowing any concurrently executing queries.
        </para>
        <para>

doc/src/sgml/ref/checkpoint.sgml

@@ -37,7 +37,7 @@ CHECKPOINT
   </para>
 
   <para>
-   The <command>CHECKPOINT</command> command forces an immediate
+   The <command>CHECKPOINT</command> command forces a fast
    checkpoint when the command is issued, without waiting for a
    regular checkpoint scheduled by the system (controlled by the settings in
    <xref linkend="runtime-config-wal-checkpoints"/>).

doc/src/sgml/ref/pg_basebackup.sgml

@@ -500,8 +500,9 @@ PostgreSQL documentation
       <term><option>--checkpoint={fast|spread}</option></term>
       <listitem>
        <para>
-        Sets checkpoint mode to fast (immediate) or spread (the default)
+        Sets checkpoint mode to fast or spread
         (see <xref linkend="backup-lowlevel-base-backup"/>).
+        The default is spread.
        </para>
       </listitem>
      </varlistentry>