Add note about the strict-binding-parser in data-binding section

[TimvdLippe]

In the section https://www.polymer-project.org/2.0/docs/devguide/data-binding#anatomy-of-a-data-binding, the docs should be expanded with a note stating there is a strict-binding-parser (introduced in Polymer/polymer#4768) that can handle more cases, but is less performant.

For an example note, see Polymer/polymer@89ab738

[twin-tigon]

Hi, I'd like to work on this one

[TimvdLippe]

Feel free to send a PR and tag me as reviewer!