Generic/DocComment: add XML documentation

[rodrigoprimo]

Description

This PR adds the XML documentation for the Generic.Commenting.DocComment sniff.

This sniff contains several errors, so I opted to add a <standard> block at the top summarizing all the checks.

Several of the valid examples are the same, but what is being checked felt different enough to use different <standard> and <code_comparison> blocks. If we want to make this documentation shorter, we might group related errors in the same <standard> and <code_comparison> blocks. For example, the errors long and short descriptions must start with a capital letter, and there must be a blank line between different doc comment elements.

Suggested changelog entry

Add XML documentation for the Generic.Commenting.DocComment 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.
• [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.
• [x] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added XML documentation for the sniff.