[x/tour] pic: document package
[x/tour] pic: document package

Effective Go¹ says every package should have a package comment,
and every exported name in a package should have a doc comment.

This package is mentioned in the Go tour. Let's set a good example.

¹ https://golang.org/doc/effective_go.html#commentary

Change-Id: Iac561c7530fc49d5ff17c51d925151ec8319ef24
Reviewed-on: https://go-review.googlesource.com/c/tour/+/232865
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
X-Tour-Commit: 8ec2108c3ab568fa335edadc5cd9a2c5ab5361ec

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website