Documentation: format-patch --root clarifications

Users were confused about the meaning and use of the --root option.
Notably, since 68c2ec7 (format-patch: show patch text for the root
commit, 2009-01-10), --root has nothing to do with showing the patch
text for the root commit any more.

Shorten and clarify the corresponding paragraph in the DESCRIPTION
section, document --root under OPTIONS, and add an explicit note that
root commits are formatted regardless.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -39,15 +39,11 @@ There are two ways to specify which commits to operate on.
    REVISIONS" section in linkgit:git-rev-parse[1]) means the
    commits in the specified range.
 
-A single commit, when interpreted as a <revision range>
-expression, means "everything that leads to that commit", but
-if you write 'git format-patch <commit>', the previous rule
-applies to that command line and you do not get "everything
-since the beginning of the time".  If you want to format
-everything since project inception to one commit, say "git
-format-patch \--root <commit>" to make it clear that it is the
-latter case.  If you want to format a single commit, you can do
-this with "git format-patch -1 <commit>".
+The first rule takes precedence in the case of a single <commit>.  To
+apply the second rule, i.e., format everything since the beginning of
+history up until <commit>, use the '\--root' option: "git format-patch
+\--root <commit>".  If you want to format only <commit> itself, you
+can do this with "git format-patch -1 <commit>".
 
 By default, each output file is numbered sequentially from 1, and uses the
 first line of the commit message (massaged for pathname safety) as
@@ -170,6 +166,13 @@ not add any suffix.
 	applied.  By default the contents of changes in those files are
 	encoded in the patch.
 
+--root::
+	Treat the revision argument as a <revision range>, even if it
+	is just a single commit (that would normally be treated as a
+	<since>).  Note that root commits included in the specified
+	range are always formatted as creation patches, independently
+	of this flag.
+
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