[Documentation] PSR12 - Open Tag

[dingo-d]

The PR contains the documentation for the PSR12/Files/OpenTag 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 OpenTag 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]

Suggested change

	When the opening <?php tag is on the first line of the file, it must be on its own line with no other statements unless it is a file containing markup outside of PHP opening and closing tags.
	When the opening <?php tag is on the first line of the file, it must be on its own line with no other statements, unless it is a file containing markup outside of PHP opening and closing tags.

[jrfnl]

@dingo-d Thanks for this PR.
Saw one missing comma, but I'm fine with leaving that until the PSR12 specs have been updated to include it.