improve code docs in api2

Note: API methos should be declared pub, so that they show up in the generated docu.

src/api2/tape/mod.rs

@@ -1,3 +1,5 @@
+//! Tape Backup Management
+
 use proxmox::api::router::SubdirMap;
 use proxmox::api::Router;
 use proxmox::list_subdirs_api_method;