Update PROTOCOL-PDO.md

[hsmade]

-1 doesn't exist in UINT, so I converted them to what the value would be in the respective UINTs

[denmaus]

I must admit that at least 4294967295 looks like a magic number.
Is not possible to use a more intuitive and clear syntax: 0xFF for 8 bit and 0xFFFF for 16 bit numbers?

[michaelarnauts]

I agree with @denmaus, while you are technically correct that it's impossible to specify a negative number with an unsigned int, it does make it more clear what the values mean by just indicating a negative number.

I prefer this clarity. Maybe a note below the table to indicate this might be better?

[hsmade]

Maybe: -1 (0xFFFF) ?

[michaelarnauts]

Maybe: -1 (0xFFFF) ?

That's also fine I guess :)