Include merge method link in attribute description

Currently, the project visibility methods attribute description link to
the descriptions and options for visibility methods. The merge methods
attribute description doesn't should do the same.

doc/api/projects.md

@@ -727,7 +727,7 @@ POST /projects
 | `public_builds` | boolean | no | If `true`, jobs can be viewed by non-project-members |
 | `only_allow_merge_if_pipeline_succeeds` | boolean | no | Set whether merge requests can only be merged with successful jobs |
 | `only_allow_merge_if_all_discussions_are_resolved` | boolean | no | Set whether merge requests can only be merged when all the discussions are resolved |
-| `merge_method` | string | no | Set the merge method used |
+| `merge_method` | string | no | Set the [merge method](#project-merge-method) used |
 | `lfs_enabled` | boolean | no | Enable LFS |
 | `request_access_enabled` | boolean | no | Allow users to request member access |
 | `tag_list`    | array   | no       | The list of tags for a project; put array of tags, that should be finally assigned to a project |
@@ -784,7 +784,7 @@ POST /projects/user/:user_id
 | `public_builds` | boolean | no | If `true`, jobs can be viewed by non-project-members |
 | `only_allow_merge_if_pipeline_succeeds` | boolean | no | Set whether merge requests can only be merged with successful jobs |
 | `only_allow_merge_if_all_discussions_are_resolved` | boolean | no | Set whether merge requests can only be merged when all the discussions are resolved |
-| `merge_method` | string | no | Set the merge method used |
+| `merge_method` | string | no | Set the [merge method](#project-merge-method) used |
 | `lfs_enabled` | boolean | no | Enable LFS |
 | `request_access_enabled` | boolean | no | Allow users to request member access |
 | `tag_list`    | array   | no       | The list of tags for a project; put array of tags, that should be finally assigned to a project |
@@ -841,7 +841,7 @@ PUT /projects/:id
 | `public_builds` | boolean | no | If `true`, jobs can be viewed by non-project-members |
 | `only_allow_merge_if_pipeline_succeeds` | boolean | no | Set whether merge requests can only be merged with successful jobs |
 | `only_allow_merge_if_all_discussions_are_resolved` | boolean | no | Set whether merge requests can only be merged when all the discussions are resolved |
-| `merge_method` | string | no | Set the merge method used |
+| `merge_method` | string | no | Set the [merge method](#project-merge-method) used |
 | `lfs_enabled` | boolean | no | Enable LFS |
 | `request_access_enabled` | boolean | no | Allow users to request member access |
 | `tag_list`    | array   | no       | The list of tags for a project; put array of tags, that should be finally assigned to a project |