Add more details to valid expression locations

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We should document how bindings in comments, `` tags, and `` 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.

Find the documentation section covering valid expression locations and review the listed behavior for comments, , , <textarea>, and tag-name positions. Verify the textarea note before editing, then document each case so the page reflects which bindings render, update, throw, or break templates.

Written by the indexing model from the issue text.