feat(topology/uniform_space/cauchy): sequentially complete space with a countable basis is complete (#1761)
* feat(topology/uniform_space/cauchy): sequentially complete space with a countable basis is complete
This is a more general version of what is currently proved in
`cau_seq_filter`. Migration of the latter file to the new code will be
done in a separate PR.
* Add docs, drop unused section vars, make arguments `U` and `U'` explicit.
* Update src/topology/uniform_space/cauchy.lean
Co-Authored-By: sgouezel <sebastien.gouezel@univ-rennes1.fr>
* Fix some comments