docs: update and improve docs for input, ngModelController, and guide/forms

[Narretz]

In this PR:

• make it more clear that you should use $validators for custom validation (updated example in guide etc)
• adds explicit information what certain inputs require (Date object for input[date] etc)
• update outdated information (css class list, parse errors)
• explain how to override built-in validators (for ye olde Email regex discussion)

Feeback welcome, I probably have introduced typos and odd wordings.

[gkalpak]

This might be slightly misleading (i.e. giving the impresssion that if the original value is restored, the control wouldn't be $dirty). Maybe "has been interacted with" ?
(Just a thought - not even sure it makes it more clear.)

[gkalpak]

@Narretz:
The lines are too long in guide/forms.ngdoc (which makes it more difficult to read, review and comment).
Since you touched this, how about fixing line lengths as well ? :)

[gkalpak]

rejected when invalid <-- probably meant rejected when invalid

[gkalpak]

+1 @Narretz for a much needed docs update !

[IgorMinar]

I read half-way through the changes and they look good. Feel free to merge when you think you are done with edits.

[Narretz]

Just merged it in. Thanks @gkalpak, @Blesh and @IgorMinar