fixed typos and minor copy edits

made some minor copy edits and fixed some typos.
removed references to 'ArcGIS API for JavaScript proxies' (because that JS API doc actually points to our resource-proxies)
also narrowed up a few code snippets as they were blowing out the column width in a few pages (most notably the methods section of [this](http://esri.github.io/esri-leaflet/api-reference/layers/image-map-layer.html) page