docs: fb: Remove matroxfb scrollback boot option

In commit 50145474 ("fbcon: remove soft scrollback code"), the
framebuffer scrollback mode was removed, but the documentation was not
updated.  Properly update the documentation by removing the option that
is no longer pressent, and removed the particular scrollback option.
Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/e5187e70faf8237825a142313802983a6c214baa.1600333774.git.unixbhaskar@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/fb/matroxfb.rst

@@ -317,8 +317,6 @@ Currently there are following known bugs:
  - interlaced text mode is not supported; it looks like hardware limitation,
    but I'm not sure.
  - Gxx0 SGRAM/SDRAM is not autodetected.
- - If you are using more than one framebuffer device, you must boot kernel
-   with 'video=scrollback:0'.
  - maybe more...
 
 And following misfeatures: