Documentation: update to git-merge-base --octopus

Unlike plain merge-base, merge-base --octopus only requires at least one
commit argument; update the synopsis to reflect that.

Add a sentence to the discussion that when --octopus is used, we do expect
'2' (the common ansestor across all) as the result.

Signed-off-by: Vincent van Ravesteijn <vfr@lyx.org>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-base.txt

@@ -9,7 +9,8 @@ git-merge-base - Find as good common ancestors as possible for a merge
 SYNOPSIS
 --------
 [verse]
-'git merge-base' [-a|--all] [--octopus] <commit> <commit>...
+'git merge-base' [-a|--all] <commit> <commit>...
+'git merge-base' [-a|--all] --octopus <commit>...
 'git merge-base' --independent <commit>...
 
 DESCRIPTION
@@ -89,6 +90,9 @@ and the result of `git merge-base A M` is '1'.  Commit '2' is also a
 common ancestor between 'A' and 'M', but '1' is a better common ancestor,
 because '2' is an ancestor of '1'.  Hence, '2' is not a merge base.
 
+The result of `git merge-base --octopus A B C` is '2', because '2' is
+the best common ancestor of all commits.
+
 When the history involves criss-cross merges, there can be more than one
 'best' common ancestor for two commits.  For example, with this topology:
 

builtin/merge-base.c

@@ -23,7 +23,8 @@ static int show_merge_base(struct commit **rev, int rev_nr, int show_all)
 }
 
 static const char * const merge_base_usage[] = {
-	"git merge-base [-a|--all] [--octopus] <commit> <commit>...",
+	"git merge-base [-a|--all] <commit> <commit>...",
+	"git merge-base [-a|--all] --octopus <commit>...",
 	"git merge-base --independent <commit>...",
 	NULL
 };