[mlir]: Clarify docs for external OpTrait::FunctionLike ops

The documentation claims that an op with the trait FunctionLike has a
single region containing the blocks that corresponding to the body of
the function. It then goes on to say that the absence of a region
corresponds to an external function when, in fact, this is represented
by a single empty region. This patch changes the wording in the
documentation to match the implementation.

Signed-off-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Klas Segeljakt <klasseg@kth.se>

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D89868