Add a <note> about the shells that require a `rehash' command to look

for new executables in PATH, after a new port has been installed.

PR:		docs/39747
Submitted by:	Chris Pepper <pepper@rockefeller.edu>

en_US.ISO8859-1/books/handbook/ports/chapter.sgml

@@ -796,6 +796,20 @@ R-deps: </screen>
 	    <command>make install</command> as two separate steps.</para>
 	</note>
 
+	<note>
+	  <para>Some shells keep a cache of the commands that are available in
+	    the directories listed in the <envar>PATH</envar> environment
+	    variable, to speed up lookup operations for the executable file of
+	    these commands.  If you are using one of these shells, you might
+	    have to use the <command>rehash</command> command after installing
+	    a port, before the newly installed commands can be used.  This is
+	    true for both shells that are part of the base-system (such as
+	    <command>tcsh</command>) and shells that are available as ports
+	    (for instance,
+	    <filename role="package">shells/bash</filename> and
+	    <filename role="package">shells/zsh</filename>).</para>
+	</note>
+
         <note>
           <para>Please be aware that the licenses of a few ports do not
             allow for inclusion on the CDROM.  This could be because a