Bug#1027954: cppad: binary-all FTBFS

[s3v]

Dear Maintainer,

I think debian/libcppad-doc.docs needs to be adjusted to reflect changes in
documentation file names:

- doc.omh was renamed to doc.xrst [1] which was then renamed tocppad.xrst [2] which
  was then renamed to user_guide.xrst [3]

- omh directory was renamed to xrst [1]

Kind Regards

[1] https://github.com/coin-or/CppAD/commit/f054e4abb22551dd3685f95e587137f78a5c9504
[2] https://github.com/coin-or/CppAD/commit/278d8dbf63b42130f25acf03d5dbefdf6200a107
[3] https://github.com/coin-or/CppAD/commit/5a139ae40b5ee0bde2474a5fc0c17ead3365effb