input: Documentation: corrections for gameport-programming.rst

Use "E.g." instead of "Eg.".
Use correct index for buttons[] array.
Update all of struct gameport's descriptions.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20210302223523.20130-5-rdunlap@infradead.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>