Generic/ArrayIndent: add XML documentation

[rodrigoprimo]

Description

This PR adds the XML documentation for the Generic.Arrays.ArrayIndent sniff.

I left some inline comments below highlighting decisions I made while creating this documentation that I'm not sure about.

Suggested changelog entry

Add XML documentation for the Generic.Arrays.ArrayIndent 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.

[rodrigoprimo]

I opted to use opening square bracket/parenthesis here and in a few other places as the sniff supports both array syntaxes. That is not super correct as in the old syntax, the opening token of an array is not just a parenthesis, but actually array(.

[jrfnl]

What about using the phrase "open brace" (and "close brace" for the end, of course) ? Would that work ? What do you think ?

[rodrigoprimo]

For this <standard> block, I'm specifically mentioning multi-line arrays. It is the only error that could apply to a single-line array, but single-line arrays are not handled by this sniff. The example that I have in mind is:

    $a = 
[1, 2];