Adjust and wordsmith Search documentation

Summary:
Ref T12450. General adjustments:

• Try to make "Cluster: Search" more about "stuff in common + types" instead of pretty much all being Elastic-specific, so we can add Solr or whatever later.
• Provide guidance about rebuilding indexes after making a change.
• Simplify the basic examples, then provide a more advanced example at the ed.
• Really try to avoid suggesting anyone configure Elasticsearch ever for any reason.

Test Plan: Read documents, previewed in remarkup.

Reviewers: chad, 20after4

Reviewed By: 20after4

Maniphest Tasks: T12450

Differential Revision: https://secure.phabricator.com/D17602