Make brief mention of quirks in keyCode and key for the different renderers

[benfry]

Apropos of processing/processing#3827 and processing/processing#3888, we're stuck with some unfixable differences between how renderers handle keyCode in particular, but also key (in the case of ENTER and RETURN).

We can't document every last difference, but we should briefly mention that behavior for key codes that we don't document (those not mentioned in PConstants), the behavior may change slightly between renderers.

See those two issues for more details about it. We could also make it a Wiki page if we need to get into that much detail. But I'd lean toward saying "keyCode is tricky across renderers and different operating systems... your mileage may (will) vary and just be careful as you switch renderers and operating systems and use undocumented keys".

[REAS]

I've added this to the keyCode and key references. @JakubValtar, please confirm that we no longer have support for PAGE_UP, PAGE_DOWN, HOME, and END. That was in the keyCode reference and I just removed it.

[JakubValtar]

@REAS Currently this is what we have:

The keys included in the ASCII specification (BACKSPACE, TAB, ENTER, RETURN, ESC, and DELETE) do not require checking to see if they key is coded, and you should simply use the key variable instead of keyCode

If you want more keys like pageup, pagedown, home, end, or F1-F12, it's trivial to add them, they are all in extended ascii.

[benfry]

I don't think we ever had support for PAGE_UP, PAGE_DOWN, HOME, and END. I think it was a reference typo that wasn't actually included in PConstants.

I don't really want to add others, we can just point people to the docs on adding more: the JavaFX docs for its constants, the Java2D docs for its own, the JOGL ones for theirs. They're all over the place and I think we support the cases used by the vast majority of our users, which is what we're always trying to do.

[REAS]

OK, I'm closing this out. Thanks gents.