Update: Improve directory recursion and its documentation.

Update: Improve directory recursion and its documentation.

Fix some problems in the `fl_directory_do()` documentation.
Extend the documentation adding more details of the process.

Remove redundant "is not error" status checks.

The checks like `if (flag_actions[j])` and `if (!flag_actions[j])` are now changed to respect the recently added `f_directory_recurse_do_flag_action_e` flag.