fix(mangen): Render the synopsis from override_usage

The SYNOPSIS was always derived from the argument list, which collapses a
command documenting several invocation forms into a single line. A
command that declared them through `Command::override_usage` ended up
with a manual page whose synopsis contradicted the rest of the text: the
`ln` utility of uutils/coreutils, for instance, listed one form while its
after-help referred to the 1st through 4th forms.

Render the synopsis from `Command::override_usage` when it is set, one
line per form, and keep deriving it from the arguments otherwise.

Fixes uutils/coreutils#12366 and #6466