rebase docs: fix incorrect format of the section Behavioral Differences

The text body of section Behavioral Differences is typeset as code,
but should be regular text. Remove the indentation to achieve that.

While here, prettify the language:

- use "the x backend" instead of "x-based rebase";
- use present tense instead of future tense;

and use subsections instead of a list.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -549,24 +549,28 @@ Other incompatible flag pairs:
 BEHAVIORAL DIFFERENCES
 -----------------------
 
- * empty commits:
+There are some subtle differences how the backends behave.
 
-    am-based rebase will drop any "empty" commits, whether the
-    commit started empty (had no changes relative to its parent to
-    start with) or ended empty (all changes were already applied
-    upstream in other commits).
+Empty commits
+~~~~~~~~~~~~~
+
+The am backend drops any "empty" commits, regardless of whether the
+commit started empty (had no changes relative to its parent to
+start with) or ended empty (all changes were already applied
+upstream in other commits).
 
-    merge-based rebase does the same.
+The merge backend does the same.
 
-    interactive-based rebase will by default drop commits that
-    started empty and halt if it hits a commit that ended up empty.
-    The `--keep-empty` option exists for interactive rebases to allow
-    it to keep commits that started empty.
+The interactive backend drops commits by default that
+started empty and halts if it hits a commit that ended up empty.
+The `--keep-empty` option exists for the interactive backend to allow
+it to keep commits that started empty.
 
-  * directory rename detection:
+Directory rename detection
+~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-    merge-based and interactive-based rebases work fine with
-    directory rename detection.  am-based rebases sometimes do not.
+The merge and interactive backends work fine with
+directory rename detection.  The am backend sometimes does not.
 
 include::merge-strategies.txt[]