I've been thinking the same thing, but for source code in general. I like that Serenity code is easy to read and rarely needs comments, but some out-of-the-way documentation could be really nice. Documenting things really helps me to understand them.
My only objection is that it might be a little premature. Things are still changing pretty quickly.

In the meantime, I would highly recommend using some form of search tool to scan the Serenity libraries. This alias is my talisman:

alias sg='grep -rnIP --color=auto --exclude-dir=Toolchain --exclude-dir=Root --include=\*.h --include=\*.cpp'

Usually I will sg <PCRE> in Libraries/ to find an API I need or in Applications/ to find usage.

alias sg='grep -rnIP --color=auto --exclude-dir=Toolchain --exclude-dir=Root --include=\*.h --include=\*.cpp'

This might be what I need and thank you for the tip :D

My only objection is that it might be a little premature. Things are still changing pretty quickly.

I think that a simple dictionary like documentation would be doable at this stage. It will contain only the data structures, the general algorithms( e.g quick_sort, binary_search ) and the tools (e.g Bitmap) that are available and should be used when possible. The dictionary can point to the specific source file for further reference.