[mlir] Fix missing OpInterface docs newline

Fix incorrect markdown generated by mlir-tblgen for an InterfaceMethod
that includes a body.  Previously, this would cause the next method to
show up on the same line and produce incorrect markdown.  Newlines would
only be added if the method did _not_ provide a body.  E.g., previously
this was generating markdown like:

    some function comment#### `next method`

This change makes this generate as:

    some function comment

    #### `next method`

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D140590