docs: use explicit framework param instead of *args

[benhoyt]

A while back we decided to recommend using an explicit framework parameter instead *args to a charm's __init__. We've updated charmcraft init templates, but we forgot a few places in our docs. Update them to match here.

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[benhoyt]

test-real-pebble failed once here. It looks like test_checks_and_health is timing-sensitive and should be improved. Re-running for now, and if it happens again we should open an issue and fix.

[tonyandrewmeyer]

Thanks!

[dwilding]

Nice one, thanks!