Improve documentation on Fields lookup methods

[lukaseder]

There has been a lot of confusion in the past regarding the behaviour of the various Fields lookup methods. The fields type has been extracted with #11031 in jOOQ 3.15, though the problem is not specific to this extraction. Except now, we have a single place to document this.

Examples of such confusion include (list might be amended):

• Select from DSLContext#innerJoin() produces incorrect results when no match #5352

While it will be impossible to change the behaviour compatibly, it will definitely be helpful to add more documentation.

[lukaseder]

Fixed in jOOQ 3.18.0 and 3.17.5 (#13997). The backport generates too many conflicts in 3.16 for it to be viable.