Minor tweaks to WMTS provider.

* Clean up documentation slightly.
* Rename `tms` constructor parameter to `tileMatrixSetID` to avoid abbreviation.
* Use a USGS WMTS server in the example in the docs instead of the French one, because the USGS one does not require a key.
* Add WMTS example to Sandcastle "Imagery Layers Manipulation" example.
Merge pull request #2022 from AnalyticalGraphicsInc/wmtsTweaks

Minor tweaks to WMTS provider.