Added documentation about GVARs, PIDFF Controllers, & Integer Math

[trailx]

• Added information about integer math
• Added divide-by-0 information
• Fix table for 'Set GVAR' row
• Added numerical limits of GVARs
• Added helpful information about PIDFF controllers

Fixes #10322

[sensei-hacker]

Looks great, thanks. What do you think of rewording this:
IPF uses non-floating-point math, so it only can return integers. If your programming line returns a decimal, it will be reduced an integer.

To this:
IPF uses integer math. If your programming line returns a decimal, it will be truncated to an integer.

[trailx]

Thanks for the feedback, I made the requested change.

[sensei-hacker]

Thanks for doing this! Merging.