Here is a script that will print out all the comment lines it finds in each file that ends in "-help.pd" in the current directory or the path given to it as an argument, like: ./list_pd_help_text.lua /usr/lib/pd-extended > list.txt
The resulting list.txt can then be searched for keywords.
Comments are printed in top-to-bottom order for the main patch and for each subpatch, but they still might not make sense out of context; this is more useful for finding appropriate objects using keywords.
I've tested it on linux and WinXp (cygwin). I used luarocks to install the lfs module. On WinXp I used lfs from http://sourceware.org/cygwinports/
While it could probably be implemented as a pd_lua object it might hang Pd if it were to print everything to the console.
Martin