Improve the English help text for the node label field to make it clear how to...

Improve the English help text for the node label field to make it clear how to separate multiple labels. This does not fix the issue fully as it needs translating still. HUDSON-3521.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@21067 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/resources/hudson/model/Node/help-labelString.html

 <div>
   Labels (AKA tags) are used for grouping multiple slaves into one logical group.
+  Use spaces between each label. For instance 'regression java6' will assign a
+  node the labels 'regression' and 'java6'.
 
   <p>
   For example, if you have multiple Windows slaves and you have jobs that require