chore(jd): improve PlayerConnectionValidateLoginEvent

[Timongcraft]

This fact seems easy to miss and since now someone else in the Discord had the same problem, this should be documented.

[Warriorrrr]

This is something I've considered adding before directly to the javadoc of getConnection, but listing what classes you can cast it to (what some could call a guarantee) seems contradictory to this note here:

Note, the type of this connection is not guaranteed to be stable across versions.

Would like to ask @Owen1212055 for his opinion on this

[Owen1212055]

This api is very version specific, which is the issue.... as if mojang changes their protocol we cannot maintain compatability for people who are relying on certain behavior.

I agree with possibly improving the documentation, but we need to slap a big "this is as of X minecraft version, and may change".

[Timongcraft]

should the tag be added to checkstyle or should I change the javadoc?