Somewhere else in this forum, one of the authors (REAS IIRC) said, with reason, that the reference listing is quite long, so they put there only the standalone functions (and classes).

mask() is, actually, somePImageObject.mask(). So it is listed in the PImage page. There are a number of other classes with specific methods not listed in the reference, only in the class' page.

Funnily, I have been bitten by that myself, in blend intensity where someone mentioned mask() and I wasn't able to find it...

Not sure what is the right way of handling this (compromise between size of page/manageability and completeness): a super-complete page A secondary page listing all methods of classes not listed in the current reference

I have no dogmatic opinion there...
I never go to the Abridged reference and indeed I am not sure it is really useful.
At "worse", you might define two classes (styles) of links: regular and advanced/extended, so beginners skimming the reference might skip visually "scary" functions.
And perhaps a "super-advanced" class for methods...

I like how the reference page is currently structured, and the fact I can switch to alphabetical listing, although I rarely do that, I prefer to rely on the incremental find feature of Firefox.
Actually, I was thinking making a Greasemonkey script like the Javadoc incremental search (but simpler!), possibly finding a solution to integrate it to the official page (has to be more portable across browsers, of course).

Maybe you can make the current extended reference the "abridged" one, and the page with all (most, for official Java classes) methods the new "extended" one.

I only provide thoughts/ideas, I leave to you the delicate task of making the choices!