[FIX] developer: rewrite a bunch of outdated info in JS reference

Previously, a lot of information on the JavaScript reference page was
outdated.

This commit rewrites a bunch of sections to bring them up to date and
removes others that are no longer relevant. While a more profound rework
of the JS documentation is still needed, this commit at least reduces
the amount of outdated information significantly in the meantime.

Author: sdegueldre