doc, test: Improve setup_clean_chain documentation
Summary:
The first commit improves documentation on setup_clean_chain which is misunderstood quite frequently. Most importantly it fixes the TestShell docs which are simply incorrect. The second commit removes the instances of setup_clean_clain in functional tests where it is not changing the default.
Backport of core#21042.
Test Plan:
ninja check-functional-extended
In test/functional, run:
git grep 'self.setup_clean_chain = False'
and check the is no occurrence in the test outside of the framework itself.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D10853