Create documentation for Turbo Fire (suggested documentation added)

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Documentation for usage of Turbo Fire function should be created. RetroArch>Settings>Input>Turbo Fire. I can't seem to get the Turbo Fire to work, but I'm not sure I'm using it correctly or understand it's intended function due to the lack of documentation. Description of the general intended function of the Turbo Fire, Turbo Modes, and Turbo Default Button would be much appreciated. There seems to be a lot of confusion in the community about how Turbo Fire is intended to work.

The most info I was able to find are some comments in the source code (see below).

if (turbo_mode > INPUT_TURBO_MODE_CLASSIC)
{
/* Pressing turbo button toggles turbo mode on or off.
* Holding the button will
* pass through, else the pressed state will be modulated by a
* periodic pulse defined by the configured duty cycle.

else if (turbo_mode == INPUT_TURBO_MODE_SINGLEBUTTON_HOLD &&
/* If turbo button is held, all buttons pressed except
* for D-pad will go into a turbo mode. Until the button is
* released again, the input state will be modulated by a
* periodic pulse defined by the configured duty cycle.

Start with the Turbo Fire settings path in RetroArch and the source-code comments included in the issue. Document the intended behavior of Turbo Fire, Turbo Modes, and Turbo Default Button, including how the listed modes respond to pressing or holding the turbo button; done means users can understand and use these settings.