docs: rewrite the bots and crawlers section in the caching guide (#98782)
## Summary
The Bots and crawlers section in the caching guide said that all bots
and crawlers skip the static shell because "they need a complete
document". Only HTML-limited bots do, and the reason is metadata: with
Partial Prerendering, `generateMetadata` can resolve dynamically and
stream in after the shell, which puts the tags in the body rather than
the `<head>`. Bots that need them in the `<head>` get a dynamic render
instead.
The second paragraph now states the consequence directly: if the
build-time prerender read something unreachable at request time, the
render throws and those bots get a 500. Which user agents this covers,
and the `htmlLimitedBots` option for changing the list, stay owned by
the Streaming guide, which the section links to.
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