Document token filter middleware

[evan-forbes]

          I would also add here: documenting of IBC's token filter middleware

Originally posted by @cmwaters in #1257 (comment)

in #1219, we added a token filter to prevent tokens outside of the native one to be transfered via IBC, this change needs to be documented in:

• the specs
• developer focused documentation such as our doc site
• internally, such as readme for https://github.com/celestiaorg/celestia-app/tree/main/x/tokenfilter or a small ADR

[cmwaters]

linking celestiaorg/docs#518 as it is relevant