Improve Javadoc of ObjectProvider to clarify what is unique

[quaff]

No description provided.

[jhoeller]

I put an explanation at the type-level javadoc instead since uniqueness primarily matters for {@link #getObject()} (or even direct injection point declarations without any ObjectProvider involved at all), with getIfUnique just being a special-purpose method that turns a non-unique case into a null return value. Also, I've worded it a bit differently. Thanks for the pull request, in any case!