Parameterization docs to fix.

[ericmjl]

I am creating this issue to keep track of what distributions have precision/variance parameterization, and whether I've put in PRs for the docs or not.

Check boxes to remind myself that they have been included in PR #2789.

• Normal
• LogNormal
• HalfNormal
• StudentT
• HalfStudentT
• AR
• GaussianRandomWalk

This list is compiled from #2786, in which @ColCarroll grepped for get_tau_sd. Happy to have more added to the list. I will also reference exact PR.

Example from @fonnesbeck:

(1) Change docstring

sd : array of floats
        the component standard deviations (only required if tau is not specified)
tau : array of floats
        the component precision (only required if sd is not specified)

(2) Add Example

Examples
--------
with pm.Model():
    x = pm.Normal('x', mu=0, sd=10)