Create Squiz ClassDeclarationStandard.xml

[richard-hp]

Added some missing documentation

Description

This is just some house-keeping to keep the docs up to date. I had to spend some time reviewing the php code to figure out what this rule did so I'm recording that for future users to save them time.

Suggested changelog entry

I just added 1 file

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
  • [ ] This change is only breaking for integrators, not for external standards or end-users.
• [x] Documentation improvement

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• [ ] I have added tests to cover my changes.
• [ ] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added XML documentation for the sniff.
• [ ] [Required for new files] I have added any new files to the package.xml file.