[spec/function.dd] Add scope parameter section

[ntrel]

• Tweak existing wording
• Mention @safe & -dip1000 needed for enforcement

Update:

• Split existing return ref example.

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[ntrel]

@thewilsonator Sorry I forgot about this. Rebased! (At least I get CircleCI notifications now when my builds fail :-) ).

[ntrel]

The scope parameter changes were getting a bit big for the function parameter table, so I have moved them to their own section and split out an example from the return scope docs.