Add more details to valid expression locations

[justinfagnani]

We should document how bindings in comments, <template> tags, and <style> when polyfilled work or don't work.

• Bindings in comments don't break a template, but don't render
• Bindings in <template> will break a template (due to them being invisible to lit-html's template prep code). Use static templates instead.
• Bindings in <style> work in native shadow DOM, but in Shady CSS they will only render the first value and never update.
• Bindings in <textarea> will work for initial render (TODO verify, and we might throw soon), but if the user edits the value the binding will break.
• Bindings in open tag name position will throw in dev mode and break a template in prod mode. Bindings in close tag name position will break a template in prod mode.