Add documentation for nginx_reloads_milliseconds metrics

[salonichf5]

Proposed changes

Problem: Documentation for nginx_reloads_milliseconds metrics is missing in NGF docs.

Solution: Add documentation for nginx_reloads_milliseconds.

Testing: TODO

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #1861

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

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please add a brief note that summarizes the change.