Documentation for unsafe_html is insufficient

[Hixie]

https://dart-lang.github.io/linter/lints/unsafe_html.html doesn't give any reason why these patterns are bad.

Even as someone who wrote the specification for many of these Web features, I literally could not tell you why some of these are so bad that we should lint them entirely out of existence (especially with an unignorable lint).

[pq]

/cc @srawlins

[gmpassos]

The lint says that is unsafe to set the href of an AnchorElement. But what’s is the correct (secure) way to do this? Will be nice to have it in the docs.