Adds proper documentation for pagerduty_tag resource in tag_assignmen…

[Coolomina]

Addresses #562

[johncoleman83]

Hi @imjaroiswebdev, I have reviewed this doc change, and looks good to me.

To provide some context, The PagerDuty terraform docs for pagerduty_tag_assignment recommend putting the type as a supported argument as @Coolomina mentions clearly in the linked issue; however the pagerduty_tag resource does not support type as an argument (see docs here). I also notice that none of the PagerDuty terraform resources (which is not the same as resources in the REST API that do support the type attribute as a specifier of the resource itself) support the type argument unless it is used to distinguish a variety of the resource rather than the resource itself.

With that said, I think these changes should be added. I suppose I should clarify are the docs at /website/docs the proper docs for PagerDuty resource that we publish here?

[imjaroiswebdev]

@johncoleman83 You totally right on what you say and @Coolomina thank you so much for your contribution, this update will definitely help other users of the Provider 💪🏽 😎