[Users] Docs on filtering/processing incomplete

[Jim Kyle]

As a user of email going back some 20 years, from Eudora through The
Bat! to Thunderbird and now claws-mail, I'm finding the descriptions of
setting up sub-folders, with filtering and processing, far from
complete. It took three days of searching for me to discover that the
essential step of clicking OK after the final "Add" of a new rule to a
list had been left out -- and it was only by sheer accident that I
discovered the "matcherrc" file in $HOME/.claws-mail that holds the
rules!

Can we improve this a bit for the benefit of others?

-- 
Jim