patch 9.1.1417: missing info about register completion in complete_info()

Problem:  missing info about register completion in complete_info()
          (after v9.1.1408)
Solution: update documentation and mention that register is used as
          source, add a test (glepnir)

closes: #17389

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_ins_complete.vim

@@ -4641,6 +4641,9 @@ func Test_register_completion()
   call feedkeys("a\<C-X>\<C-R>\<Esc>", 'tx')
   call assert_equal("zero", getline(1))
 
+  call feedkeys("Sze\<C-X>\<C-R>\<C-R>=string(complete_info(['mode']))\<CR>\<ESC>", "tx")
+  call assert_equal("zero{'mode': 'register'}", getline(1))
+
   " Clean up
   bwipe!
   delfunc GetItems

src/version.c

@@ -709,6 +709,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1417,
 /**/
     1416,
 /**/