doc SYNOPSIS & -h: use "-" to separate words in labels, not "_"

doc SYNOPSIS & -h: use "-" to separate words in labels, not "_"

Change "builtin/credential-cache--daemon.c" to use "<socket-path>" not
"<socket_path>" in a placeholder label, almost all of our
documentation uses this form.

This is now consistent with the "If a placeholder has multiple words,
they are separated by dashes" guideline added in
9c9b4f2f8b7 (standardize usage info string format, 2015-01-13), let's
add a now-passing test to assert that that's the case.

To do this we need to introduce a very sed-powered parser to extract
the SYNOPSIS from the *.txt, and handle not all commands with "-h"
having a corresponding *.txt (e.g. "bisect--helper"). We'll still want
to handle syntax edge cases in the *.txt in subsequent commits for
other checks, but let's do that then.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>