docs(ruleset): document required_reviewers rule parameter

Add documentation for the new required_reviewers nested block
in both organization_ruleset and repository_ruleset resources.

Documents the reviewer, file_patterns, and minimum_approvals
attributes with their types and requirements.

Author: deiga

website/docs/r/organization_ruleset.html.markdown

@@ -211,6 +211,22 @@ The `rules` block supports the following:
 
 * `required_review_thread_resolution` - (Optional) (Boolean) All conversations on code must be resolved before a pull request can be merged. Defaults to `false`.
 
+* `required_reviewers` - (Optional) (Block List) Require specific reviewers to approve pull requests. Note: This feature is in beta. (see [below for nested schema](#rules.pull_request.required_reviewers))
+
+#### rules.pull_request.required_reviewers ####
+
+* `reviewer` - (Required) (Block List, Max: 1) The reviewer that must review matching files. (see [below for nested schema](#rules.pull_request.required_reviewers.reviewer))
+
+* `file_patterns` - (Required) (List of String) File patterns (fnmatch syntax) that this reviewer must approve.
+
+* `minimum_approvals` - (Required) (Number) Minimum number of approvals required from this reviewer. Set to 0 to make approval optional.
+
+#### rules.pull_request.required_reviewers.reviewer ####
+
+* `id` - (Required) (Number) The ID of the reviewer (Team ID).
+
+* `type` - (Required) (String) The type of reviewer. Currently only `Team` is supported.
+
 #### rules.required_status_checks ####
 
 * `required_check` - (Required) (Block Set, Min: 1) Status checks that are required. Several can be defined. (see [below for nested schema](#rules.required_status_checks.required_check))

website/docs/r/repository_ruleset.html.markdown

@@ -217,6 +217,22 @@ The `rules` block supports the following:
 
 * `required_review_thread_resolution` - (Optional) (Boolean) All conversations on code must be resolved before a pull request can be merged. Defaults to `false`.
 
+* `required_reviewers` - (Optional) (Block List) Require specific reviewers to approve pull requests. Note: This feature is in beta. (see [below for nested schema](#rules.pull_request.required_reviewers))
+
+#### rules.pull_request.required_reviewers ####
+
+* `reviewer` - (Required) (Block List, Max: 1) The reviewer that must review matching files. (see [below for nested schema](#rules.pull_request.required_reviewers.reviewer))
+
+* `file_patterns` - (Required) (List of String) File patterns (fnmatch syntax) that this reviewer must approve.
+
+* `minimum_approvals` - (Required) (Number) Minimum number of approvals required from this reviewer. Set to 0 to make approval optional.
+
+#### rules.pull_request.required_reviewers.reviewer ####
+
+* `id` - (Required) (Number) The ID of the reviewer (Team ID).
+
+* `type` - (Required) (String) The type of reviewer. Currently only `Team` is supported.
+
 #### rules.required_deployments ####
 
 * `required_deployment_environments` - (Required) (List of String) The environments that must be successfully deployed to before branches can be merged.