Document that Instants are truncated to the millisecond when indexed

Microseconds/Nanoseconds are ignored when an Instant is indexed.

This was true in Search 5 as well.

This is because:

1. Elasticsearch does the same: https://github.com/elastic/elasticsearch/issues/10005 and we didn't try to work around that by adding additional fields.
2. The Lucene backend uses a long to represent the instant, so we can't go beyond millisecond precision without reducing the span of indexable instants. Or we would need to index instants differently.

We may want to do better in the future, but I suspect doing better will also impact performance, so maybe we'll need to make that configurable.