unused_imports/F401: Explain when imports are preserved (#9963)
The docs previously mentioned an irrelevant config option, but were
missing a link to the relevant `ignore-init-module-imports` config
option which _is_ actually used.
Additionally, this commit adds a link to the documentation to explain
the conventions around a module interface which includes using a
redundant import alias to preserve an unused import.
(noticed this while filing #9962)
