cmd/internal/cmdconfig: fill in experiment descriptions
cmd/internal/cmdconfig: fill in experiment descriptions

If an experiment lacks a description in the config file, use
the description from internal.Experiments.

Log an error if the experiment name is not in that map.

Change-Id: I3362d9a71d963a202d1616b9f2c6cf347314332c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260117
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite