There is no single way of searching through everything. You are advised to use a combination of different methods and information sources.
The following is what I am doing, it's the way that works best for me. It doesn't necessarily mean that it's also the best for everyone.
The PDF manuals
I downloaded all available manuals from the Pragma site and the ConTeXtGarden and divided the usefull ones (columns, details, etc.) from the ones I don't need (aleph, calculat, etc.). This directory can easily be searched through. (However, I know the content, I don't need to search.)
The mailing list
I either search locally in my mail client or I go to the Gmane search where it's very convenient to search through various mailing lists (e.g.
Whenever I need to search the source I fire up a script that changes to the source directory and sets handy options for
alias cgrep='grep \
You can easily perform searches which output the file and the line number of the relevant files. Searches are fastest if started from the directory
$ cgrep '\def\startFLOWchart' *
And of course the search box in the ConTeXt wiki offers a good service for searching the wiki (and other sites).