[lit-html] Docs should describe lit-html change detection explictly

Nobody has claimed this yet.

Currently the docs say things like, "When you call render, lit-html only updates the parts of the template that have changed since the last render. This makes lit-html updates very fast."

But they don't say explicitly how this happens. In particular, that lit keeps track of the current value at each binding site (aka "hole in the DOM", aka Part). On render, it compares the new value with the current rendered value, and only re-renders if they're different.

The `live()` directive is designed to apply different change detection logic (against the live DOM value, instead of the value that lit rendered).

No file or test is named. Search the documentation for the quoted change-detection wording, then review how the text describes binding values, Parts, and the live() directive. Done means the docs explicitly explain the comparison with the current rendered value and distinguish live() behavior.

Written by the indexing model from the issue text.