LUCENE-9662: Update concurrent index checking usage instructions

[zacharymorn]

Description

Update concurrent index checking usage instructions

Sample output

> Task :lucene:core:CheckIndex.main() FAILED

ERROR: index path not specified
Usage: java org.apache.lucene.index.CheckIndex pathToIndex [-exorcise] [-slow] [-segment X] [-segment Y] [-threadCount X] [-dir-impl X]

  -exorcise: actually write a new segments_N file, removing any problematic segments
  -fast: just verify file checksums, omitting logical integrity checks
  -slow: do additional slow checks; THIS IS VERY SLOW!
  -codec X: when exorcising, codec to write the new segments_N file with
  -verbose: print additional details
  -segment X: only check the specified segments.  This can be specified multiple
              times, to check more than one segment, eg '-segment _2 -segment _a'.
              You can't use this with the -exorcise option
  -threadCount X: number of new threads created and used to check index concurrently.
                  When not specified, this will default to the number of CPU cores up to 4.
                  When '-threadCount 1' is used, index checking will be performed sequentially.
  -dir-impl X: use a specific FSDirectory implementation. If no package is specified the org.apache.lucene.store package will be used.

**WARNING**: -exorcise *LOSES DATA*. This should only be used on an emergency basis as it will cause
documents (perhaps many) to be permanently removed from the index.  Always make
a backup copy of your index before running this!  Do not run this tool on an index
that is actively being written to.  You have been warned!

Run without -exorcise, this tool will open the index, report version information
and report any exceptions it hits and what action it would take if -exorcise were
specified.  With -exorcise, this tool will remove any segments that have issues and
write a new segments_N file.  This means all documents contained in the affected
segments will be removed.

This tool exits with exit code 1 if the index cannot be opened or has any
corruption, else 0.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Lucene maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.

[mikemccand]

Thanks @zacharymorn -- I left a couple small comments.

[mikemccand]

Maybe just say number of threads used to check index concurrently? I.e. drop the "new" because then I start to wonder if main thread counts :)

[zacharymorn]

Ah yes sorry :D . Updated the instruction.

[mikemccand]

Maybe we should change this default to not cap at 4? Just use number of cores? This is command-line execution, which is typically done only once at a time (versus when CheckIndex is invoked from our tests...).

[zacharymorn]

Sounds good. Updated the code and instruction here.

[mikemccand]

Looks great! Thanks @zacharymorn!

[zacharymorn]

Looks great! Thanks @zacharymorn!

Thanks Michael for the review and approval!