more precise doc on makedoc

a full explanation was already there.
I added a link to it

developer-doc/usingDoxygen.txt

@@ -3,9 +3,12 @@
 
 \page usingDoxygen Creating plumed documentation
 
-<B> Whenever you make a change to the documentation for PLUMED you need to write include the phrase
+<B> Whenever you make a change to the documentation for PLUMED and you want the
+online version of the manual to be updated, you need to write include the phrase
 [makedoc] in your next commit message.  Doing so ensures that the online manuals are updated the next 
-time you push your commits to master. </B>
+time you push your commits to master.
+See more in the section \ref updating-web-manuals
+</B>
 
 To create the plumed manual you should go to the <b> user-doc </b> directory and type <b> make </b>. 
 This command works because user documentation for all the PLMD::Action is inside the source code.  If