Generators do not always respect `--sniffs` CLI argument

[jrfnl]

Describe the bug

Okay, so this is a weird one and I haven't been able to create a reliable reproduction case.

// Will show the docs for the one sniff.
$ phpcs --generator=Text --standard=Stnd --sniffs=Stnd.Category.SniffName


// Will sometimes show the docs for the one sniff, but will also sometimes show the docs for the complete standard in use (`phpcs.xml.dist`).
$ phpcs --generator=Text --sniffs=Stnd.Category.SniffName

I've tried to create a reproduction scenario to track down the issue, but I haven't been able to - other than with a new sniff in an external standard which isn't public yet.

// Shows the docs based on the `phpcs.xml.dist` file, including the new sniff (PHPCompatibility itself is included in the `phpcs.xml.dist` ruleset).
$ phpcs --generator=Text --sniffs=PHPCompatibility.ParameterValues.RemovedXmlSetHandlerCallbackUnset

// Shows the docs for the one sniff.
$ phpcs --generator=Text --standard=PHPCompatibility --sniffs=PHPCompatibility.ParameterValues.RemovedXmlSetHandlerCallbackUnset

More than anything, I'm opening this issue as a reminder to investigate and to figure out a reproduction case which reliably triggers the bug.

Expected behavior

// Show the docs for the one sniff.
$ phpcs --generator=Text --standard=Stnd --sniffs=Stnd.Category.SniffName

// Show the docs for the one sniff if the sniff is included in the ruleset PHPCS defaults to
// (`[.]phpcs.xml[.dist]` or PEAR if no custom ruleset found).
$ phpcs --generator=Text --sniffs=Stnd.Category.SniffName

// Show "No sniffs registered" if the sniff is NOT included in the ruleset PHPCS defaults to.
$ phpcs --generator=Text --sniffs=Stnd.Category.SniffName

Versions (please complete the following information)

Operating System	not relevant
PHP version	not relevant
PHP_CodeSniffer version	master
Standard	see description
Install type	git clone

Additional context

I've seen this issue on and off for years now, but haven't dug in deep to pinpoint the exact trigger as adding the --standard=Stnd CLI arg seems to fix the issue.

If I remember correctly, I've predominantly seen this behaviour when testing docs for external standards. Not for docs for any of the PHPCS native standards. (which is part of the reason why creating a reproduction case is more tricky)

Please confirm

• I have searched the issue list and am not opening a duplicate issue.
• I have read the Contribution Guidelines and this is not a support question.
• I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.
• I have verified the issue still exists in the master branch of PHP_CodeSniffer.