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>

Documentation/CodingGuidelines

@@ -650,8 +650,8 @@ Writing Documentation:
    (One or more of <file>.)
 
  Optional parts are enclosed in square brackets:
-   [<extra>]
-   (Zero or one <extra>.)
+   [<file>...]
+   (Zero or more of <file>.)
 
    --exec-path[=<path>]
    (Option with an optional argument.  Note that the "=" is inside the
@@ -665,6 +665,16 @@ Writing Documentation:
    [-q | --quiet]
    [--utf8 | --no-utf8]
 
+ Use spacing around "|" token(s), but not immediately after opening or
+ before closing a [] or () pair:
+   Do: [-q | --quiet]
+   Don't: [-q|--quiet]
+
+ Don't use spacing around "|" tokens when they're used to seperate the
+ alternate arguments of an option:
+    Do: --track[=(direct|inherit)]
+    Don't: --track[=(direct | inherit)]
+
  Parentheses are used for grouping:
    [(<rev> | <range>)...]
    (Any number of either <rev> or <range>.  Parens are needed to make