Add note in the manual about recovery near retention policy

This commit Closes#200. It is adding a note to the recovery
section in the documentation manual alerting the user about
the possibility of the backup getting obsolete while the
recovery process is ongoing and what to do to prevent that.

The team first thought about a solution that could handle this
automatically but many arguments were against this, specially
because it could introduce unwanted behaviour on recover and
the fact that `keep` feature is there to help.

References: BAR-222

Signed-off-by: Andre <andre.marchesini@enterprisedb.com>

doc/manual/43-backup-commands.en.md

@@ -198,6 +198,16 @@ command.
 
 The recovery command has several options that modify the command behavior.
 
+> **IMPORTANT:**
+> If you're unsure whether the backup you want to recover from will be
+> automatically deleted by retention policies during recovery,
+> mark it issuing the [keep command](#keep) using the appropriate target.
+> Once the recovery process is complete and your new PostgreSQL instance
+> has reached the desired recovery target, if you don’t want to
+> keep the backup beyond the retention policy, you can remove `keep`
+> annotation issuing the [keep command](#keep) again using the
+> `--release` option.
+
 ### Remote recovery
 
 Add the `--remote-ssh-command <COMMAND>` option to the invocation