docs should explain difference between 'full sniff' and 'partial sniff'

Problem/Motivation

This issue #2994956: Excluding sniffs from phpcs is resolved with this explanation:

> With the --exclude option you can only exclude full sniffs like Drupal.Commenting.FunctionComment. You cannot exclude parts like Drupal.Commenting.FunctionComment.MissingParamType.

But this doesn't exaplin what is a full sniff and what is a part sniff -- is it 3 pieces in the name vs 4?

The docs page at https://www.drupal.org/docs/contributed-modules/code-review-module/php-c... needs to explain this, otherwise you see a sniff in a report such as 'Drupal.Files.TxtFileLineLength.TooLong', try to use it with a --sniffs or --exclude flag and you get:

> ERROR: The specified sniff code "Drupal.Files.TxtFileLineLength.TooLong" is invalid

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes