Hey Larry, use the 3rd Advanced Filters tab to search for anything written in the gametext. So if you write "knight", not only companions with the keyword 
knight will appear (which is the result you got searching for "knight" in the 1st tab), but also all cards that interact with knights: many fortifications, items like 
Gondor Bow or 
Catapult, companions like 
Denethor Steward of the City... and 
Armor of the White City. Of course, you must clear any keyword specification in the 1st tab filter, or you won't get Denethor, 
Gondor Bow, 
Catapult... (which use but don't have the "knight" keyword.)
By writting a gametext piece, you must write it exactly as it would appear in the card, word by word. For instance, if you write "
may not be overwhelmed" (never using quotation marks) you get 
Coat of Mail, 
Dwarven Armor but not 
Bounder or 
Severed His Bonds (which say "
prevent a Hobbit from being overwhelmed"). To get 
Ghan-buri-Ghan in the list, you need to write "
cannot be overwhelmed". To get them all, of course, just write "overwhelmed" or "overwhelm".
To include a culture symbol in a gametext piece, it must be written between brackets [ ]. To add twilight symbols, you need to use parenthesis ( ).
Hope this helps.