[mlir][ods] Add description to Attr and ensure doc generation

Add the description textual field to the Attr ODS class to mirror an
identical field in the Type ODS class. Add support for generating
documentation for attribute constraints defined using this field. This
ensures mlir-tblgen produces at least some documentation for dialects
that only define attribute constraints, such as DLTI.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D123024