[wtp-pmc] Cleaned up a bit of planning document

[David M Williams <david_williams@xxxxxxxxxx>]

As promised, I've added reference links
to our "exception process" and also to the "compatibility
tests" document. 

See http://wiki.eclipse.org/index.php/Web_Tools_Platform_Project_2.0_Plan#Compatibility_with_Other_WTP_Releases


And ... did  several other minor
editing changes. Let me know if questions,  or if anything is not
quite right. 

But, ready for Rhagu's follow up note,
as far as I am concerned. 

Thanks,