Maint: Fix borked help/man output for node clean

This action's help was busted:

• You can't put bulleted lists in the description string of an action. It blows up the man page formatting.
• Description must begin with a description of the action, not a continuation of the summary; otherwise it makes the output of 'puppet help cmd action' useless.
• The description of how unexport works wasn't sufficiently clear.