update pylab_import_all help string

IPython/core/shellapp.py

@@ -176,8 +176,11 b' class InteractiveShellApp(Configurable):'
         """
     )
     pylab_import_all = Bool(True, config=True,
-        help="""If true, an 'import *' is done from numpy and pylab,
-        when using pylab"""
+        help="""If true, IPython will populate the user namespace with numpy, pylab, etc.
+        and an 'import *' is done from numpy and pylab, when using pylab mode.
+        
+        When False, pylab mode should not import any names into the user namespace.
+        """
     )
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC')