docs: document repo corruption recovery, refs #9825

[mr-raj12]

Description

Documentation for items 2-4 of the status update in #9825.

• borg check help: an item's chunk id list always refers to the correct chunks, so a missing chunk that a later borg create stores again makes all archives referencing it complete again, without another borg check --repair.
• borg recreate help: reworded the hint about missing chunks accordingly.
• FAQ, "Repairing a damaged repository": added the order of operations after repository corruption, including removing permanently lost files with borg recreate --exclude.

Item 1 (borg compact reporting missing objects) is not part of this PR.

refs #9825

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.11%. Comparing base (7e3d305) to head (fd560dc).
⚠️ Report is 7 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10507      +/-   ##
==========================================
- Coverage   89.12%   89.11%   -0.01%     
==========================================
  Files         103      103              
  Lines       19783    19783              
  Branches     3099     3099              
==========================================
- Hits        17631    17630       -1     
- Misses       1490     1492       +2     
+ Partials      662      661       -1     

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