[haiku-doc] API guideline update

Hi guys,

As promissed, the API guidelines have been updated to be in sync with
the coding guidelines. I expect this to be the last change of this
sort, unless someone sees more inconsistencies.

The changes in short:
- Use commas as separator between names and email addresses
- Use tabs to indent. Tab length 4.
- There should be two whitelines between blocks.

If there are any comments, let me know.

Niels

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