git-rebase.txt: update note about directory rename detection and am

In commit 6aba117 ("am: avoid directory rename detection when
calling recursive merge machinery", 2018-08-29), the git-rebase manpage
probably should have also been updated to note the stronger
incompatibility between git-am and directory rename detection.  Update
it now.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Johannes Sixt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

Documentation/git-rebase.txt

@@ -570,8 +570,9 @@ it to keep commits that started empty.
 Directory rename detection
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The merge and interactive backends work fine with
-directory rename detection.  The am backend sometimes does not.
+Directory rename heuristics are enabled in the merge and interactive
+backends.  Due to the lack of accurate tree information, directory
+rename detection is disabled in the am backend.
 
 include::merge-strategies.txt[]