docs(config): clarify scope of directory filter_mode

[tessus]

Currently the table leaves the impression that the directory mode is limited to the current host, since from top to bottom is global, host, and session. Session is obviously limited to the host, thus leaves the next filter a bit ambiguous. Is it also limited to the host?

This change clarifies the scope of the filter.

Mode	Description
global (default)	Search history from all hosts, all sessions, all directories
host	Search history just from this host
session	Search history just from the current session
directory	Search history just from the current directory (global)

P.S.: I've just noticed that my editor also got rid of superfluous whitespace characters.

[tessus]

I wanted to bring this up again.

But if you think it is clear that directory is a global search, please feel free to close this PR.