[Docs] update docs to reflect implementation of respiration signal cleaning with khodadad2018

[danibene]

Description

See #950 (comment).

Proposed Changes

1. I changed the docstring of the rsp_clean() function so that it describes the filter used.
2. I changed rsp_methods() so that the information for the processing report also describes the filter used.

Checklist

Here are some things to check before creating the PR. If you encounter any issues, do let us know :)

• I have read the CONTRIBUTING file.
• My PR is targeted at the dev branch (and not towards the master branch).
• I ran the CODE CHECKS on the files I added or modified and fixed the errors.
• I have added the newly added features to News.rst (if applicable)