multibodyplant: tell scenegraph my name when registering as a source

[RussTedrake]

As discussed with @gizatt and @SeanCurtis-TRI in #13971. The change does seem painless, and only an improvement.

---

This change is 

[RussTedrake]

+@SeanCurtis-TRI for feature review, please.

Reviewable status: LGTM missing from assignee SeanCurtis-TRI(platform), needs at least two assigned reviewers (waiting on @SeanCurtis-TRI)

[SeanCurtis-TRI]

One caveat -- I think it's worth calling out somewhere in documentation (probably the documentation for RegisterAsSourceForSceneGraph) that the plant's name will be passed to SceneGraph where it propagates through to other domains (e.g., visualization), therefore, it is highly recommended to give the plant a recognizable name; the default name won't always be convenient.

I also wonder about the utilities that generate the plant/scene graph pair and what those names look like.

Reviewed 1 of 1 files at r1.
Reviewable status: needs at least two assigned reviewers

[RussTedrake]

Done (Added to the doc).

Those utilities you refer to do explicitly set the names "plant" and "scene_graph". https://github.com/RobotLocomotion/drake/blob/master/multibody/plant/multibody_plant.cc#L3039

Reviewable status: needs at least two assigned reviewers (waiting on @SeanCurtis-TRI)

[SeanCurtis-TRI]

Reviewed 1 of 1 files at r2.
Reviewable status: needs at least two assigned reviewers

[RussTedrake]

+@rpoyner-tri for platform review, please; per the schedule.

Reviewable status: LGTM missing from assignee rpoyner-tri(platform) (waiting on @rpoyner-tri)

[rpoyner-tri]

Reviewed 1 of 1 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all discussions resolved, LGTM from assignees rpoyner-tri(platform),SeanCurtis-TRI(platform)