Reapply completion docs prefetch (#22306)

Leaving release notes as N/A because it had release notes in the past in
#21705

In #21286, documentation resolution was made more efficient by only
resolving the current completion. However, this meant that single line
documentation shown inline in the menu was missing until scrolled
to. This also meant that it would wait for navigation to resolve
completion docs, leading to lag for displaying documentation.

This change resolves this by attempting to fetch all the completions
that will be shown. It also mostly avoids re-resolving completions. It
intentionally re-resolves the current selection on navigation, as some
language servers will respond with more information later on.

Release Notes:

- N/A