Fix doc about 'default' fg/bg color

[egmont]

The manual page says:

"The possible colors are: [...] And there is a special keyword for transparent background. It is 'default'. The 'default' can only be used for background color."

Apparently this restriction is not true, the word 'default' can stand either at the foreground or at the background color position (or even both), meaning the default fg or bg color of the terminal emulator, respectively.

We should rephrase the above sentence (which would probably imply omitting the word "transparent" and go for the "terminal emulator's default" instead).

(We should double check that it works in ncurses too, I've checked with slang.)

This is useful e.g. in order to say this in a skin file:

[core]
    commandlinemark = default;default;reverse