[Documentation] PSR12 - Declare Statement

[dingo-d]

The PR contains the documentation for the PSR12/Files/DeclareStatement sniff.

Description

This PR will add the documentation for the above-mentioned sniff, according to the official standard definitions.

Suggested changelog entry

Add documentation for the PSR12 DeclareStatement sniff

Related issues/external references

Part of #148

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.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• 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.

[jrfnl]

@dingo-d FYI: I'm going to park this PR for the time being. This sniff needs a complete rewrite, which I started a while back, but never got round to finishing. The intention is to keep the error messages and codes the same with the rewrite, but I'd rather verify the docs once I've finished the rewrite if you don't mind.

[jrfnl]

Closing - see: #148 (comment)