last: (man) reduce an inflated table to sane proportions

last: (man) reduce an inflated table to sane proportions

Also, reword some things for better flow, improve some markup, and
combine username and tty to [username|tty]... in the synopsis, as
this better indicates that the two identifiers can be mixed.

Furthermore, use quotes around the time formats that contain a space,
as otherwise the time would be understood as a user name / tty name.
Drop the "tomorrow" and "+5min" examples, as they don't make sense
here.  And change the "-5days" example to a true format that covers
most possibilities, and give a few examples of it after the table.

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