Street Level: shorter comments and docs

- Comments in the action, the layer, embed, the Mapillary policy and the
  controls say why in fewer words; restatements of the code are gone.
- CURRENT-STATE's "Show me X on Street View" section is a list by topic and
  no longer says the browser gate aims at coverageLines.
- TOOLS.md's street-level view paragraph is shorter.

docs/TOOLS.md

@@ -104,18 +104,16 @@ draws them once the link has been restored.
 
 A view can open street-level photos too: `street_level` names a source
 (`google` for Street View, `mapillary`, `kartaview`, `panoramax`, or none for
-whichever the panel has lit) and a point, which defaults to the spot the camera looks at. Street View
+the lit one) and a point, by default the spot the camera looks at. Street View
 shows only on the Google 3D map, so a `google` source sets `map` to
-`photoreal` and any other map is refused; a view cannot both follow an entity
-and open photos. The `street-level` layer is turned on. Links carry it as
-`sl=<source>,<lat>,<lon>`; once the link has restored, the app runs its
-`show_street_level` action, which flies to the point at street framing and
-opens the nearest photo. A plain link with a `google` source stops before the
-billed load: the panel is armed and OPEN GOOGLE STREET VIEW is one click away.
-A view posted into the embedded app, or the panel page's own opening link,
-opens the photo, since it is the client's own request; `?embed=1` on a
-published link does not. Mapillary, KartaView and Panoramax links open the
-photo at once; none of them bills per image. A link that follows an entity ignores `sl`.
+`photoreal` and refuses another; a view cannot both follow an entity and open
+photos. Links carry it as `sl=<source>,<lat>,<lon>`: once restored, the app
+runs `show_street_level`, which flies there and opens the nearest photo. A
+plain link with a `google` source only arms the panel for OPEN GOOGLE STREET
+VIEW; a view posted into the embedded app, or the panel page's own opening
+link, opens it, and `?embed=1` on a published link does not. Mapillary,
+KartaView and Panoramax, which do not bill per image, open at once. A link
+that follows an entity ignores `sl`.
 
 Tools take a view as `VIEW_ARGUMENTS`: an `area` to frame from above, or a
 `camera`, plus `layers`, `style`, `map`, `follow`, `annotations` and