math.big, docs: document the behaviour of % for negative numbers; in V: -10 % 7 == -3 (#24604)

Author: spytheman

doc/docs.md

@@ -8293,6 +8293,9 @@ Assignment Operators
 &&= ||=
 ```
 
+Note: in V, `assert -10 % 7 == -3` passes. In programming, the sign of the remainder
+depends upon the signs of divisor and dividend.
+
 ## Other online resources
 
 ### [V contributing guide](https://github.com/vlang/v/blob/master/CONTRIBUTING.md)

vlib/math/big/integer.v

@@ -470,6 +470,8 @@ pub fn (dividend Integer) / (divisor Integer) Integer {
 //
 // WARNING: this method will panic if `divisor == 0`. For a modular division method that
 // returns a Result refer to `mod_checked`.
+// Note: in V, `assert big.integer_from_i64(-10) % big.integer_from_i64(7) == big.integer_from_i64(-3)` passes.
+// In other words, the result is negative 3, and is NOT positive 4.
 @[inline]
 pub fn (dividend Integer) % (divisor Integer) Integer {
 	if dividend.signum == -1 {