rebase: fix docs about incompatibilities with --root

In commit 5dacd4abdd ("git-rebase.txt: document incompatible options",
2018-06-25), I added notes about incompatibilities between options for
the apply and merge backends.  Unfortunately, I inverted the condition
when --root was incompatible with the apply backend.  Fix the
documentation, and add a testcase that verifies the documentation
matches the code.

While at it, the documentation for --root also tried to cover some of
the backend differences between the apply and merge backends in relation
to reapplying cherry picks.  The information:
  * assumed that the apply backend was the default (it isn't anymore)
  * was written before --reapply-cherry-picks became an option
  * was written before the detailed information on backend differences
All of these factors make the sentence under --root about reapplying
cherry picks contradict information that is now available elsewhere in
the manual, and the other references are correct.  So just strike this
sentence.

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

Documentation/git-rebase.txt

@@ -574,10 +574,7 @@ See also INCOMPATIBLE OPTIONS below.
 --root::
 	Rebase all commits reachable from `<branch>`, instead of
 	limiting them with an `<upstream>`.  This allows you to rebase
-	the root commit(s) on a branch.  When used with `--onto`, it
-	will skip changes already contained in `<newbase>` (instead of
-	`<upstream>`) whereas without `--onto` it will operate on every
-	change.
+	the root commit(s) on a branch.
 +
 See also INCOMPATIBLE OPTIONS below.
 
@@ -656,7 +653,7 @@ are incompatible with the following options:
  * --reapply-cherry-picks
  * --edit-todo
  * --update-refs
- * --root when used in combination with --onto
+ * --root when used without --onto
 
 In addition, the following pairs of options are incompatible:
 

t/t3422-rebase-incompatible-options.sh

@@ -65,6 +65,10 @@ test_rebase_am_only () {
 		test_must_fail git rebase $opt --update-refs A
 	"
 
+	test_expect_success "$opt incompatible with --root without --onto" "
+		git checkout B^0 &&
+		test_must_fail git rebase $opt --root A
+	"
 }
 
 # Check options which imply --apply