Add a test and doc

runtime/doc/eval.txt

@@ -2302,6 +2302,8 @@ range({expr} [, {max} [, {stride}]])
 				List	items from {expr} to {max}
 readfile({fname} [, {binary} [, {max}]])
 				List	get list of lines from file {fname}
+register_executing()		Number	|TRUE| if the register executing
+register_recording()		String	get the recording register name
 reltime([{start} [, {end}]])	List	get time value
 reltimefloat({time})		Float	turn the time value into a Float
 reltimestr({time})		String	turn time value into a String

src/testdir/test_functions.vim

@@ -899,3 +899,28 @@ func Test_trim()
   let chars = join(map(range(1, 0x20) + [0xa0], {n -> nr2char(n)}), '')
   call assert_equal("x", trim(chars . "x" . chars))
 endfunc
+
+" Test for register_recording() and register_executing()
+func Test_register_executing_and_recording()
+  let s:reg_stat = ''
+  func s:save_reg_stat()
+    let s:reg_stat = register_recording() . ':' . register_executing()
+    return ''
+  endfunc
+
+  new
+  call s:save_reg_stat()
+  call assert_equal(':0', s:reg_stat)
+  call feedkeys("qa\"=s:save_reg_stat()\<CR>pq", 'xt')
+  call assert_equal('a:0', s:reg_stat)
+  call feedkeys("@a", 'xt')
+  call assert_equal(':1', s:reg_stat)
+  call feedkeys("qb@aq", 'xt')
+  call assert_equal('b:1', s:reg_stat)
+  call feedkeys("q\"\"=s:save_reg_stat()\<CR>pq", 'xt')
+  call assert_equal('":0', s:reg_stat)
+
+  bwipe!
+  delfunc s:save_reg_stat
+  unlet s:reg_stat
+endfunc