Fix Generalized Harmonic Documentation

[nilsdeppe]

Bug reports:

• [ ] The notation should match what is in #1494
• [ ] The notation in C++ comments should match #1494
• [ ] Should be able to find documentation starting from Topics menu
• [ ] gr::Tags namespace should show up in the General Relativity topic in Doxygen.
• [ ] gr::Tags::SpacetimeChristoffelFirstKind has no documentation.
• [ ] gr::Tags::SpacetimeChristoffelSecondKind has no documentation.
• [ ] gr::Tags::TraceSpacetimeChristoffelFirstKind has no documentation.

[nilsdeppe]

ping @geoffrey4444