patch 8.1.0072: use of 'termwinkey' is inconsistent

Problem:    Use of 'termwinkey' is inconsistent.
Solution:   Change the documentation and the behavior. (Ken Takata)

runtime/doc/terminal.txt

@@ -87,7 +87,8 @@ to the job.  For example:
	'termwinkey' CTRL-W    move focus to the next window
	'termwinkey' :	       enter an Ex command
	'termwinkey' 'termwinkey' send 'termwinkey' to the job in the terminal
	'termwinkey' .	       send a CTRL-W to the job in the terminal
	'termwinkey' .	       send 'termwinkey' to the job in the terminal
	'termwinkey' CTRL-\    send a CTRL-\ to the job in the terminal
	'termwinkey' N	       go to terminal Normal mode, see below
	'termwinkey' CTRL-N    same as CTRL-W N
	'termwinkey' CTRL-C    same as |t_CTRL-W_CTRL-C|

src/terminal.c

@@ -2107,7 +2107,11 @@ terminal_loop(int blocking)
    in_terminal_loop = curbuf->b_term;

    if (*curwin->w_p_twk != NUL)
    {
	termwinkey = string_to_key(curwin->w_p_twk, TRUE);
	if (termwinkey == Ctrl_W)
	    termwinkey = 0;
    }
    position_cursor(curwin, &curbuf->b_term->tl_cursor_pos);
    may_set_cursor_props(curbuf->b_term);

@@ -2203,12 +2207,13 @@ terminal_loop(int blocking)
		/* "CTRL-W CTRL-C" or 'termwinkey' CTRL-C: end the job */
		mch_signal_job(curbuf->b_term->tl_job, (char_u *)"kill");
	    }
	    else if (termwinkey == 0 && c == '.')
	    else if (c == '.')
	    {
		/* "CTRL-W .": send CTRL-W to the job */
		c = Ctrl_W;
		/* "'termwinkey' .": send 'termwinkey' to the job */
		c = termwinkey == 0 ? Ctrl_W : termwinkey;
	    }
	    else if (termwinkey == 0 && c == Ctrl_BSL)
	    else if (c == Ctrl_BSL)
	    {
		/* "CTRL-W CTRL-\": send CTRL-\ to the job */
		c = Ctrl_BSL;

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    72,
/**/
    71,
/**/