(GH-258) Clarify `Invoke-DscResource` is incompatible with composite resources

[michaeltlombardi]

PR Summary

Prior to this change, the reference documentation for Invoke-DscResource didn't clearly state that the cmdlet is incompatible with composite resources.

This change:

• Clarifies that Invoke-DscResource can't be used to invoke composite resources, which require the LCM.
• Resolves #258
• Fixes AB#255214

PR Checklist

• [x] Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• [x] Summary: This PR's summary describes the scope and intent of the change.
• [x] Contributor's Guide: I have read the contributors guide.
• [x] Style: This PR adheres to the style guide.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 8b7579f:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
dsc/dsc-1.1/PSDesiredStateConfiguration/Invoke-DscResource.md	:white_check_mark:Succeeded	View (dsc-1.1)
dsc/dsc-2.0/PSDesiredStateConfiguration/Invoke-DscResource.md	:white_check_mark:Succeeded	View (dsc-2.0)
dsc/dsc-3.0/PSDesiredStateConfiguration/Invoke-DscResource.md	:white_check_mark:Succeeded	View (dsc-3.0)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel