hterm: tweak custom mouse support

After playing around with this behavior a bit more, it turns out that
it is not easy to handle the fallback keyword for the user.  The CSS
syntax allows for a single fallback keyword, but only when url() is
used.  That means if the user went with a keyword instead (e.g.
'crosshair'), or added their own fallback, we'd set the cursor to a
value like 'crosshair, auto' which is invalid and Chrome would ignore
it completely.

Update the FAQ to note the fallback requirement accordingly.

Change-Id: Ibd84eb0456f1c4c06e652808e2f445b83a01225c
Reviewed-on: https://chromium-review.googlesource.com/1112565
Reviewed-by: Vitaliy Shipitsyn <[email protected]>
Tested-by: Mike Frysinger <[email protected]>

js/hterm_scrollport.js

@@ -307,7 +307,7 @@ hterm.ScrollPort.prototype.decorate = function(div) {
       'height: 100%;' +
       'width: 100%;' +
       'overflow: hidden;' +
-      'cursor: var(--hterm-mouse-cursor-style), auto;' +
+      'cursor: var(--hterm-mouse-cursor-style);' +
       '-webkit-user-select: none;' +
       '-moz-user-select: none;');