hterm: allow F11 when running in a tab for fullscreen
hterm: allow F11 when running in a tab for fullscreen

Since fullscreen mode also allows us to capture Ctrl+W and such, document
it explicitly for tabs and extension usage where "open as window" is not
an option.

Change-Id: Ia4052f26c596262bbb4630ce6f457df2a308642a
Reviewed-on: https://chromium-review.googlesource.com/844015
Reviewed-by: Brandon Gilmore <adbfbaf7e4f927b45ce8ad3e5043a632414a833b@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm