[mlir] [linalg] Update documentation for fuse_into_containing_op

This patch updates the docs for fuse_into_containing_op. It
updates the returned values to include the new_containing_op
and adds a brief description of the new_containing_op.

updates the docs with new changes in the
op regarding the return of the new_containing_op as well
as a brief description o

Differential Revision: https://reviews.llvm.org/D152044