chore(release data source): add 'Description' fields

This adds `Description` fields to the release data source, per request of @deiga
via #2515 (comment)

Signed-off-by: Mike Ball <mikedball@gmail.com>

Author: mdb

github/data_source_github_release.go

@@ -14,33 +14,38 @@ import (
 
 func dataSourceGithubRelease() *schema.Resource {
 	return &schema.Resource{
-		Read: dataSourceGithubReleaseRead,
-
+		Read:        dataSourceGithubReleaseRead,
+		Description: "Use this data source to retrieve information about a GitHub release in a specific repository.",
 		Schema: map[string]*schema.Schema{
 			"repository": {
-				Type:     schema.TypeString,
-				Required: true,
+				Type:        schema.TypeString,
+				Required:    true,
+				Description: "Name of the repository to retrieve the release from.",
 			},
 			"owner": {
-				Type:     schema.TypeString,
-				Required: true,
+				Type:        schema.TypeString,
+				Required:    true,
+				Description: "Owner of the repository.",
 			},
 			"retrieve_by": {
-				Type:     schema.TypeString,
-				Required: true,
+				Type:        schema.TypeString,
+				Required:    true,
+				Description: "Describes how to fetch the release. Valid values are `id`, `tag`, `latest`.",
 				ValidateDiagFunc: toDiagFunc(validation.StringInSlice([]string{
 					"latest",
 					"id",
 					"tag",
 				}, false), "retrieve_by"),
 			},
 			"release_tag": {
-				Type:     schema.TypeString,
-				Optional: true,
+				Type:        schema.TypeString,
+				Optional:    true,
+				Description: "ID of the release to retrieve. Must be specified when `retrieve_by` = `tag`.",
 			},
 			"release_id": {
-				Type:     schema.TypeInt,
-				Optional: true,
+				Type:        schema.TypeInt,
+				Optional:    true,
+				Description: "ID of the release to retrieve. Must be specified when `retrieve_by` = `id`.",
 			},
 			"target_commitish": {
 				Type:     schema.TypeString,