make the manpage more clear

make the manpage more clear

some of the terms used in the manpage aren't exactly obvious, especially
to a new user. this is an attempt at making the manual more clear and
remove any potential confusion.

here's a couple issues i've noticed where users were confused due to non
clear manpage description:

https://github.com/jarun/nnn/issues/1268
https://github.com/jarun/nnn/issues/1269
https://github.com/jarun/nnn/issues/1209