gp_ functions exposed but not documented

#### Summary:
The GP Covariance functions have multiple signatures exposed, but none are listed in the documentation. These functions are:

- `gp_dot_prod_cov`
- `gp_exp_quad_cov`
- `gp_matern32_cov`
- `gp_matern52_cov`
- `gp_exponential_cov`
- `gp_periodic_cov`

#### Current Version:
v2.18.0
