Typo, link, and example fixes for treelist

Made the commands into a list and ensured their listed order is consistent

Consistent spacing after `//`

Add "the" before button names

Fix broken links, resolve redirects, and add a few links

**Only one filterable option from Grid** was mentioned, so replaced that with an example containing all `filterable` options and pointed to the Grid documentation. This way, they'll be always in sync, as features added to the Grid are backported to TreeList (see however the forum threads I raised about feature parity - [incell](http://www.telerik.com/forums/incell-edit-mode-for-treelist) and [filter via header row](http://www.telerik.com/forums/filter-via-header-row-in-treelist))

Explain *how* Content-Disposition should be set