Update documentation for search indexer and related troubleshooting steps (see #11995) (rebased onto develop)



This is the same as gh-672 but rebased onto develop.

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This set of commits adds a section to the Sysadmin troubleshooting guide covering the various steps that can be taken to fix a busted search index. It also contains commits that update and/or modify sections referred to in the added troubleshooting section.