Fix documentation of JPA spec compliance

Depending on where you look, there's currently contradicting information about the JPA specification version compliance of OpenJPA.

• The project homepage states 3.0 "targets" JPA 2.1.
• The 3.0 announcement states 3.0 is "based on" JPA 2.2.
• The About section of the reference documentation states that 3.0 implements JPA 2.0.

It would be cool if this could be clarified. I assume the latter is just an oversight and a missing update. However it would also be cool if for the former two, it could be clarified what being "based on" and "targetting" actually means.

A Twitter conversation seems to reveal that 3.0 implements 2.1 and has partial support for 2.2 features. Would be cool if that could be confirmed and documented accordingly, esp. which of the 2.2 features are supposed to work.

So to me it looks like follows:

• 2.x – implements JPA 2.0
• 3.0 – implements JPA 2.1, partial support for 2.2

The Wikipedia page on JPA could use an update on this as well. I can help with that, if you come to a decision what to phrase the current state of affairs.

Thanks and keep up the great work!