Components: better "rail" vs "track" docs in <RangeControl />

[ciampo]

Why

The RangeControl component's README has an "Anatomy" section which refers to the different elements that the component is made of.

This section is missing an entry about the "Rail" part.

What

Update the RangeControl README to add info about the "Rail" element:

• The rail represents the whole surface of the slider (from the min to the max value selectable by the user) (the part in blue in the screenshot)
• The track represents the part of the rail from the minimum value possible to the currently selected value (the part in red in the screenshot)

range-control-track-rail.mp4

A/C

• Add a "Rail" entry to the "Anatomy" section in the RangeControl component's README
• Update the "Track" entry
• Update the schematics

[kkoppenhaver]

@ciampo I've put together a PR for this over at #44057. But I see the third item in your checklist is Update the schematics and I'm not sure if there's more to be done on this one or if the PR is ok as-is.

[ryanwelcher]

@ciampo @kkoppenhaver just wanted to nudge this to see if we can bring it across the finish line.

[ciampo]

Closing this issue as it was fixed by #44057