Server hooks: explain different GL_PROTOCOL values

doc/administration/server_hooks.md

@@ -142,7 +142,7 @@ The following set of environment variables are available to server hooks.
 |:---------------------|:----------------------------------------------------------------------------|
 | `GL_ID`              | GitLab identifier of user that initiated the push. For example, `user-2234` |
 | `GL_PROJECT_PATH`    | (GitLab 13.2 and later) GitLab project path                                 |
-| `GL_PROTOCOL`        | (GitLab 13.2 and later) Protocol used with push                             |
+| `GL_PROTOCOL`        | (GitLab 13.2 and later) Protocol used for this change. One of: `http` (Git Push using HTTP), `ssh` (Git Push using SSH), or `web` (all other actions). |
 | `GL_REPOSITORY`      | `project-<id>` where `id` is the ID of the project                          |
 | `GL_USERNAME`        | GitLab username of the user that initiated the push                         |