Fix int documentation on overflow (which is UB)

[cousteaulecommandant]

The documentation says that if you add 1 to an int variable whose value is 32767 it'll just "roll over" to -32768, but this is actually undefined behavior (for signed integers of any length) and can't be trusted in behaving consistently. Example:

boolean will_overflow(int x) {
    int y = x+1;
    return y < x; // compiler assumes this will never happen => will always return false
}

    will_overflow(INT_MAX)  // returns false instead of true

Notice that this is not the case for unsigned integer types, which are required to "roll over" as documented.

[cousteaulecommandant]

Alternatively, reject this PR and instead fix Arduino so that it adds the -fwrapv flag, which makes signed integers behave in the currently documented way (which is probably easier to understand for inexperienced programmers so it might be a better option).