Restructure documentation for git-merge-base.

Restructure the text of git-merge-base to better explain more clearly
the different modes of operation.

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

Documentation/git-merge-base.txt

@@ -23,23 +23,21 @@ that does not have any better common ancestor is a 'best common
 ancestor', i.e. a 'merge base'.  Note that there can be more than one
 merge base for a pair of commits.
 
-Unless `--octopus` is given, among the two commits to compute the merge
-base from, one is specified by the first commit argument on the command
-line; the other commit is a (possibly hypothetical) commit that is a merge
-across all the remaining commits on the command line.  As the most common
-special case, specifying only two commits on the command line means
-computing the merge base between the given two commits.
+OPERATION MODE
+--------------
+
+As the most common special case, specifying only two commits on the
+command line means computing the merge base between the given two commits.
+
+More generally, among the two commits to compute the merge base from,
+one is specified by the first commit argument on the command line;
+the other commit is a (possibly hypothetical) commit that is a merge
+across all the remaining commits on the command line.
 
 As a consequence, the 'merge base' is not necessarily contained in each of the
 commit arguments if more than two commits are specified. This is different
 from linkgit:git-show-branch[1] when used with the `--merge-base` option.
 
-OPTIONS
--------
--a::
---all::
-	Output all merge bases for the commits, instead of just one.
-
 --octopus::
 	Compute the best common ancestors of all supplied commits,
 	in preparation for an n-way merge.  This mimics the behavior
@@ -52,6 +50,12 @@ OPTIONS
 	from any other.  This mimics the behavior of 'git show-branch
 	--independent'.
 
+OPTIONS
+-------
+-a::
+--all::
+	Output all merge bases for the commits, instead of just one.
+
 DISCUSSION
 ----------