replacing deprecated usage of True, False, Null validators in docs#5677

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stampycode wants to merge 4 commits intosymfony:2.7from

Commits on Sep 8, 2015

Renaming constraint rst files to Is* to preserve edit history
Tim Stamp
committed
Creating placeholder constraint rst docs for deprecated usage
Tim Stamp
committed
Updated constraint reference docs for Null->IsNull, False->IsFalse, True->IsTrue
Tim Stamp
committed
Updating book examples to not use deprecated validation methods
Tim Stamp
committed