Fix #305: update documentation of @ProviderType and @ConsumerType

Took the thread on the osgi-dev mailing list and rephrased it.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
@ProviderType: incorporate review remark of BJ

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Merge pull request #306 from fhuberts/master

Fix #305: update documentation of @ProviderType and @ConsumerType