[t:5113] carefully document a non-obvious contract requirement for change descriptor. clients may ONLY update a FT's cmp descriptor if it's the first operation done after open, before any reads or writes. further, remove the test - it's user error.


git-svn-id: file:///svn/toku/tokudb@44804 c7de825b-a66e-492c-adef-691d508d4ae1