docs(forms): fix typo
docs(forms): fix formatting
docs(compiler): fix variable scope in drag sample

Note that without this fix, if you add a second draggable element, the
two instances clobber each other since there is only one set of
startx/starty/x/y variables.
Here is an example: http://plnkr.co/edit/aGrLXcIo2SuaePuAdfmQ?p=preview.
On the surface it looks like it would be fine because you only have one
mouse but in practice the start position jumps when you start dragging.
Here it is fixed: http://plnkr.co/edit/VuvPasuumtCeiVRisYKQ?p=preview
docs(compiler): don't drag selected content

In the example with draggable, the mouseDown handler needs to start with an event.preventDefault(). Otherwise the following bug occurs:
1) Select the text of the draggable span by clicking outside the span and dragging the mouse to the left or right through the span. Release the mouse button.
2) Now click on the span's inner text, and start to Drag it. The browser's default functionality that drags highlighted text so that it can be pasted into something else (say a document in a text editor) is invoked.
3) Release the mouse button. Now suddenly, you'll be dragging the span. But you won't be able to place it down on the page. It'll just follow the mouse around until the page is refreshed.

Closes: #2465
docs(guide:unit-testing): add an example unit test for directives
docs(di): fix typos and grammar