Tweak projectPath to specify fullPath

- Clarify that project path should be the project's
fullPath rather than its relative path

doc/api/graphql/reference/gitlab_schema.graphql

@@ -3991,7 +3991,7 @@ input EpicAddIssueInput {
   issueIid: String!
 
   """
-  The project the issue belongs to
+  The full path of the project the issue belongs to
   """
   projectPath: ID!
 }

doc/api/graphql/reference/gitlab_schema.json

@@ -11115,7 +11115,7 @@
             },
             {
               "name": "projectPath",
-              "description": "The project the issue belongs to",
+              "description": "The full path of the project the issue belongs to",
               "type": {
                 "kind": "NON_NULL",
                 "name": null,

ee/app/graphql/mutations/epics/add_issue.rb

@@ -11,7 +11,7 @@ module Mutations
 
       argument :project_path, GraphQL::ID_TYPE,
                required: true,
-               description: 'The project the issue belongs to'
+               description: 'The full path of the project the issue belongs to'
 
       argument :issue_iid, GraphQL::STRING_TYPE,
                required: true,