libstdc++: Improve doxygen docs for <random>

libstdc++: Improve doxygen docs for <random>

Add @headerfile and @since tags. Add gamma_distribution to the correct
group (poisson distributions). Add a group for the sampling
distributions and add the missing definitions of their probability
functions. Add uniform_int_distribution back to the uniform
distributions group.

libstdc++-v3/ChangeLog:

	* include/bits/random.h (gamma_distribution): Add to the right
	doxygen group.
	(discrete_distribution, piecewise_constant_distribution)
	(piecewise_linear_distribution): Create a new doxygen group and
	fix the incomplete doxygen comments.
	* include/bits/uniform_int_dist.h (uniform_int_distribution):
	Add to doxygen group.