Docs: update search documentation for Tantivy backend

- configuration.md: add PAPERLESS_SEARCH_LANGUAGE and
  PAPERLESS_ADVANCED_FUZZY_SEARCH_THRESHOLD settings
- usage.md: replace Whoosh query language link with Tantivy; remove
  "inexact terms are slow" note; add full natural date keyword list;
  add fuzzy search note
- api.md: update autocomplete ordering description (alphabetical, not Tf/Idf)
- administration.md: deprecate `optimize` subcommand (now a no-op);
  add one-time reindex upgrade note

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/administration.md

@@ -459,11 +459,20 @@ document_index {reindex,optimize}
 Specify `reindex` to have the index created from scratch. This may take
 some time.
 
-Specify `optimize` to optimize the index. This updates certain aspects
-of the index and usually makes queries faster and also ensures that the
-autocompletion works properly. This command is regularly invoked by the
+Specify `optimize` to optimize the index. This command is regularly invoked by the
 task scheduler.
 
+!!! note
+
+    The `optimize` subcommand is deprecated and is now a no-op. Tantivy manages
+    segment merging automatically; no manual optimization step is needed.
+
+!!! note
+
+    On first startup after upgrading from a previous version, paperless detects
+    that the index format has changed and automatically performs a one-time full
+    reindex. No manual migration step is required.
+
 ### Clearing the database read cache
 
 If the database read cache is enabled, **you must run this command** after making any changes to the database outside the application context.