CodingGuidelines: update and clarify command-line conventions

CodingGuidelines: update and clarify command-line conventions

Edit the section which explains how to create a good SYNOPSIS section
for clarity and accuracy, it was mostly introduced in
c455bd8950e (CodingGuidelines: Add a section on writing documentation,
2010-11-04):

 * Change "extra" example to "file", which now naturally follows from
   previous "<file>..." example (one or more) to "[<file>...]" (zero or
   more).

 * Explain how we prefer spacing around "[]()" tokens and "|"
   alternatives, this is not a new policy, but just codifies what's
   already the pattern in the most wide use in the documentation.

Having a space around " | " for flags, but not for flag values is
inconsistent, but this style guide codifies existing
patterns. Grepping shows that we don't have any instance matching the
second "Don't" example:

	git grep -E -h -o '=\([^)]+\)' -- builtin Documentation/

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