Set out rules for cache-friendly conditional requests

This issue arose out of the discussion in #98.

The Search spec should reference the HTTP specification to say that servers MAY implement conditional HTTP requests using the standard (`eTag`/`If-Match` and `Date`/`If-Modified-Since`) headers to help clients take advantage of the built-in caching behaviour of HTTP.

Locate the Search specification and review the discussion in #98, then check the HTTP specification's terminology for conditional requests. Done when the Search spec states that servers may use the standard eTag/If-Match and Date/If-Modified-Since headers to support HTTP caching.

Written by the indexing model from the issue text.