git-rebase.txt: document behavioral differences between modes

There are a variety of aspects that are common to all rebases regardless
of which backend is in use; however, the behavior for these different
aspects varies in ways that could surprise users.  (In fact, it's not
clear -- to me at least -- that these differences were even desirable or
intentional.)  Document these differences.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rebase.txt

@@ -546,6 +546,38 @@ Other incompatible flag pairs:
  * --rebase-merges and --strategy
  * --rebase-merges and --strategy-option
 
+BEHAVIORAL DIFFERENCES
+-----------------------
+
+ * empty commits:
+
+    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).
+
+    merge-based rebase 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.
+
+  * empty commit messages:
+
+    am-based rebase will silently apply commits with empty commit
+    messages.
+
+    merge-based and interactive-based rebases will by default halt
+    on any such commits.  The `--allow-empty-message` option exists to
+    allow interactive-based rebases to apply such commits without
+    halting.
+
+  * directory rename detection:
+
+    merge-based and interactive-based rebases work fine with
+    directory rename detection.  am-based rebases sometimes do not.
+
 include::merge-strategies.txt[]
 
 NOTES

Documentation/technical/directory-rename-detection.txt

@@ -90,3 +90,26 @@ directory rename detection support in:
     simply not implemented.  Further, to implement this, directory rename
     detection logic would need to move from merge-recursive to
     diffcore-rename.
+
+  * am
+
+    git-am tries to avoid a full three way merge, instead calling
+    git-apply.  That prevents us from detecting renames at all, which may
+    defeat the directory rename detection.  There is a fallback, though; if
+    the initial git-apply fails and the user has specified the -3 option,
+    git-am will fall back to a three way merge.  However, git-am lacks the
+    necessary information to do a "real" three way merge.  Instead, it has
+    to use build_fake_ancestor() to get a merge base that is missing files
+    whose rename may have been important to detect for directory rename
+    detection to function.
+
+  * rebase
+
+    Since am-based rebases work by first generating a bunch of patches
+    (which no longer record what the original commits were and thus don't
+    have the necessary info from which we can find a real merge-base), and
+    then calling git-am, this implies that am-based rebases will not always
+    successfully detect directory renames either (see the 'am' section
+    above).  merged-based rebases (rebase -m) and cherry-pick-based rebases
+    (rebase -i) are not affected by this shortcoming, and fully support
+    directory rename detection.