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.
(cherry picked from commit
d711f8f81fc5f3a5a0420337f414bce93e1cad1e)