added help text

core/src/main/resources/hudson/matrix/LabelAxis/help.html

+<div>
+    This axis lets you run the same build on multiple slaves (by specifying those slaves either by their labels
+    or their individual names.) This is useful for example to cross-compile your C projects on multiple platforms,
+    testing your Java projects on different OSes, and so on.
+</div>
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core/src/main/resources/hudson/matrix/LabelExpAxis/help.html

+<div>
+    This axis lets you run the same build on multiple slaves (much like the "slaves" axis),
+    but this is more advanced in that it lets you use boolean expressions (such as <tt>foo&amp;&amp;bar</tt>)
+</div>
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