Improve Backing-up and Search Tunings articles and cover limitations when it comes to reindexing and restoring, or changing the operation mode

• Type: Improvement
• Status: In Review
• Priority: Major
• Resolution: Unresolved
• Fix Version/s: 7.2.x, 7.3.x
• Component/s: Search Infrastructure
• Labels: None
• Type of Documentation: Deployment
• Git Pull Request: https://github.com/rbohl/liferay-docs/pull/172

Things we should better cover:

• There are no database data for Search Tunings => liferay-<companyId>-search-tuning-* indexes are the primary storage
• Search Tuning entries (synonyms and result rankings) are not preserved when switching from Embedded/Sidecar to Remote/Production or when migrating to a new Elasticsearch cluster (e.g. when moving to a new major version of ES)-> Existing documents must be restored manually ==> Do not use Search Tunings when Elasticsearch is configured to run in development mode (Embedded/Sidecar)
• Reindex does not reindex Result Ranking entries, though they are not deleted either
• Synonym entries are preserved across subsequent reindexes as long as the Elasticsearch cluster remains the same
• Search Tuning modules (or DXP) must be restarted when switching to Remote/Production mode otherwise there will be index_not_found exceptions when visiting RR or SS on the Control Panel.
• Always back-up all Liferay indexes -> List OOTB indexes

Articles to update (check others):

• https://help.liferay.com/hc/en-us/articles/360029124271-Backing-up-a-Liferay-DXP-Installation
• https://learn.liferay.com/dxp/7.x/en/installation-and-upgrades/maintaining-a-liferay-dxp-installation/backing-up.html
• https://help.liferay.com/hc/en-us/articles/360034473852-Search-Tuning-Synonym-Sets
• https://help.liferay.com/hc/en-us/articles/360034473872-Search-Tuning-Customizing-Search-Results