PSR2/SwitchDeclaration: improve XML documentation

[rodrigoprimo]

Description

PSR2/SwitchDeclaration: update doc as case indentation is not checked

The sniff no longer checks case keyword indentation as of commit 4a39005, but the corresponding XML documentation was not updated. This commit aligns the documentation with the current sniff behavior by removing mentions of case indentation checks.

PSR2/SwitchDeclaration: update doc as case body should be non-empty

This commit updates the valid and invalid code examples in a <code_comparison> block, as the There must be a comment when fall-through is intentional in a non-empty case body error is only triggered for non-empty case bodies. It also fixes the positioning of the <em> tags in the modified code examples.

PSR2/SwitchDeclaration: improve XML documentation

• Documentation title should match the sniff name.
• Use different <standard> blocks for each error triggered by the
  sniff.
• Minor changes to the text to make it more clear and consistent.

PSR2/SwitchDeclaration: add missing errors to the XML documentation

This commit adds four sniff errors not previously documented to the PSR2.ControlStructures.SwitchDeclaration sniff XML documentation.

Suggested changelog entry

PSR2.ControlStructures.SwitchDeclaration: minor fixes and improvements to the XML documentation

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.