SOLR-15133: Document how to eliminate Failed to reserve shared memory warning

[epugh]

Description

The warning is annoying if you know what it is, and alarming if you don't!

Solution

Add to the Docker FAQ how to disable it.

Eventually this might feed the Ref Guide!

Tests

Doc only

Checklist

Please review the following and check all that apply:

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

[epugh]

@ctargett I'd love a quick review of the copy editing I did...

[HoustonPutman]

Looks good to me!

Will be time to migrate this stuff to the ref guide soon 😅

[ctargett]

Sorry, don't have time to review today I don't think - go ahead if Houston thinks it's good to merge.