fix: RuleDocsInfo type mismatch with eslint type

[hedgepigdaniel]

Description

Change type of RuleDocsInfo.category to disallow arrays

Fixes #2936

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

This is a type change only.

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[hedgepigdaniel]

So it seems that this change just makes way for another type error. Not sure that I have the time to keep going, though arguably this is already an improvement in that the types get closer to what eslint expects.