Denote multiple arguments with 'arg ...' not 'args'

Denote multiple arguments with 'arg ...' not 'args'

A few programs used the plural in their synopsis which doesn't read as
clear as the obvious triple-dot notation.

mdoc(7) .Ar defaults to "file ..." if no arguments are given and consistent
use of 'arg ...' matches that behaviour.

Cleanup a few markups of the same argument so the text keeps reading
naturally;  omit unhelpful parts like 'if optional arguments are given,
they are passed along' for tools like time(1) and timeout(1) that obviously
execute commands with whatever arguments where given -- just like doas(1)
which doesn't mention arguments in its DESCRIPTION in the first place.

For expr(1) the difference between 'expressions' and 'expression ...' is
crucial, as arguments must be passed as individual words.

Feedback millert jmc schwarze deraadt
OK jmc