Update documentation

* Prevent nnn from waiting on open file process

* Add env var flag to not wait for child process when opening file

* Set nowait flag once to skip bit-or every time we open a file

* Add documentation for NNN_NOWAIT

F2 is not very portable e.g., doesn't work over putty (default settings).

In certain workflows you know you would have revisit a directory. Mark it!

^Q would quit even from filter mode

1. Open directories in filter mode
2. Fix arrow keys not working after filter selection (due to invalid cur entry)
3. Support `Insert` key to clear filter prompt
4. Update documentation

Reasons:

gnome-search-tool is much more powerful than other tools in its category. It
supports numerous search parameters like size, modification date and supports
text search.