Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 m_n(Arg1)

 neutron_mass(Arg1)

 m_p(Arg1)

 proton_mass(Arg1)

 c(Arg1)

 speed_of_light(Arg1)

 b(Arg1)

 barn(Arg1)

 m_e(Arg1)

 electron_mass(Arg1)