Make all packages have a docstring

[maruel]

There's a general issue #56 but this one is focused on things that can be done as a single commit.

First, make these page nicer by making sure every package has a docstring:

• https://godoc.org/github.com/luci/gae/service
• https://godoc.org/github.com/luci/gae
• https://godoc.org/github.com/luci/gae/impl

While adding package doc to gae/service/datastore, it should include documentation for magic tags $id and $kind.