doc: Document how to undo port-select

This documents how to undo a port-select by describing the `none`
value for port-select.

Author: cvengler

doc/port-select.1.txt

@@ -20,6 +20,9 @@ SYNOPSIS
 [cmdsynopsis]
 *port* [*-dv*] *select* [--set] 'group' 'option'
 
+[cmdsynopsis]
+*port* [*-dv*] *select* [--set] 'group' none
+
 DESCRIPTION
 -----------
 *port select* provides a mechanism to choose from different implementations of
@@ -43,6 +46,9 @@ the current primary implementation and list all available implementations given
 a group name, and change the primary implementation. See *OPTIONS* below for
 details.
 
+The special value *none* undos a previous *port select* invocation by removing
+the relevant symbolic links again.
+
 OPTIONS
 -------
 *--summary*::
@@ -72,6 +78,11 @@ $> sudo port select --set mysql $option
 where '$option' is 'mysql56' if you want the version from MySQL, or 'percona',
 if you want the version from Percona.
 
+If you now want to do undo this selection, you can do so by running
+----
+$> sudo port select --set mysql none
+----
+
 SEE ALSO
 --------
 man:port[1]