Rename --windowid to --window-id; add usage warning to documentation

man/feh.pre

@@ -920,10 +920,17 @@ comes before
 Note that this option only has an effect when a sort mode is set using
 .Cm --sort .
 .
-.It Cm --windowid Ar windowid
+.It Cm --window-id Ar windowid
 .
 Draw to an existing X11 window by its ID
 .Ar windowid .
+This option is intended for use with software such as xcreensaver or
+xsecurelock, which provide a window for other applications to draw into.
+Unexpected things will happen if you specify a window belonging to software
+which does not expect
+.Nm
+to draw into it or attempt to use options or keybindings which affect window
+attributes, such as full-screen mode.
 .
 .It Cm --xinerama-index Ar screen
 .

src/help.raw

@@ -100,7 +100,7 @@ OPTIONS
      --scroll-step COUNT   scroll COUNT pixels when movement key is pressed
      --cache-size NUM      imlib cache size in mebibytes (0 .. 2048)
      --auto-reload         automatically reload shown image if file was changed
-     --windowid            Draw to an existing X11 window by its ID
+     --window-id           Draw to an existing X11 window by its ID
 
 MONTAGE MODE OPTIONS
  -X, --ignore-aspect       Set thumbnail to specified width/height without

src/options.c

@@ -433,7 +433,7 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 #endif
 		{"class"         , 1, 0, 249},
 		{"no-conversion-cache", 0, 0, 250},
-		{"windowid", 1, 0, 251},
+		{"window-id", 1, 0, 251},
 		{0, 0, 0, 0}
 	};
 	int optch = 0, cmdx = 0;