Clarify the role of checkpoint at the begininng of base backups

Output a message about checkpoint starting in verbose mode of
pg_basebackup, and make the documentation state more clearly that this
happens.

Author: Michael Banck

doc/src/sgml/backup.sgml

@@ -862,7 +862,8 @@ SELECT pg_start_backup('label', false, false);
      <xref linkend="guc-checkpoint-completion-target">).  This is
      usually what you want, because it minimizes the impact on query
      processing.  If you want to start the backup as soon as
-     possible, change the second parameter to <literal>true</>.
+     possible, change the second parameter to <literal>true</>, which will
+     issue an immediate checkpoint using as much I/O as available.
     </para>
 
     <para>

doc/src/sgml/ref/pg_basebackup.sgml

@@ -419,7 +419,7 @@ PostgreSQL documentation
       <term><option>--checkpoint=<replaceable class="parameter">fast|spread</replaceable></option></term>
       <listitem>
        <para>
-        Sets checkpoint mode to fast or spread (default) (see <xref linkend="backup-lowlevel-base-backup">).
+        Sets checkpoint mode to fast (immediate) or spread (default) (see <xref linkend="backup-lowlevel-base-backup">).
        </para>
       </listitem>
      </varlistentry>
@@ -659,6 +659,14 @@ PostgreSQL documentation
  <refsect1>
   <title>Notes</title>
 
+  <para>
+   At the beginning of the backup, a checkpoint needs to be written on the
+   server the backup is taken from.  Especially if the option
+   <literal>--checkpoint=fast</literal> is not used, this can take some time
+   during which <application>pg_basebackup</application> will be appear
+   to be idle.
+  </para>
+
   <para>
    The backup will include all files in the data directory and tablespaces,
    including the configuration files and any additional files placed in the