I'm using this snippet to test on both Windows and Linux

#include <gmp.h>

int main (void) {

	mpz_t p;

	mpz_init_set_str(p, "-1000", 10);

	mpz_nextprime(p, p);
	gmp_printf("mpz_nextprime : %Zd\n", p);

	return 0;
}

It looks like the correct answer is 2, on both windows mpir 2.5.1 and 2.6.0 and 
linux with 
gmp. The tests are also pass now, the doc is wrong.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.