freqman: limiting description size to 30, and minor fix

[gullradriel]

I refreshed the documentation and set back the limit of 30 characters per entry descriptions.

• TextEdit is also limited to 30.
• existing too long lines are untouched in file until edited.

Solves https://github.com/portapack-mayhem/mayhem-firmware/issues/1384

• Edited 'category' to only display "F: filename"

Solves https://github.com/portapack-mayhem/mayhem-firmware/issues/1363