terminal-colors.d: (man) do not show 'type' as an optional part

terminal-colors.d: (man) do not show 'type' as an optional part

That is: remove the square brackets from around 'type' in the synopsis.

Also, do not give the impression that a leading dot by itself is fine
before the 'type'.  That is: a dot is required only when 'name' and/or
'@term' is present.

Also, do not colorize the square brackets as if they were part of the
placeholders.  (And use ++double plus++ passthroughs for the opening
square brackets, to prevent asciidoctor from misinterpreting them.)

Indent the list of file types, for clarity.

And correct or improve some wordings, and remove an inconvenient
blank line in an example.

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>