[class.dd] Move Objective-C method specifics to subsection

[ntrel]

The Objective-C method specifics (added in #2540) make it hard to read the docs for people not interested in Objective-C linkage. This moves the details to a subsection.

Also:

• Mention that member functions are also called methods (which is used in some places in class.dd already).
• 2 minor tweaks to surrounding class docs.

[dlang-bot]

Thanks for your pull request and interest in making D better, @ntrel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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[thewilsonator]

Looks fine to me, cc @jacob-carlborg

[jacob-carlborg]

Fine by me.