DOC: Add remark on release functions to C-API documentation

doc/capi/index.rst.in

@@ -29,7 +29,11 @@ C API
    When working with libcaosdb's C API keep the following in
    mind. Delete all objects (transactions, entities, properties,
    parents, ...) that you created using a `caosdb_..._create_...`
-   function and only those.
+   function or released using a `caosdb_..._release_...` and only
+   those. This means, that any objects set by a `caosdb_..._get_...`
+   function, are managed by another owning object (e.g., a connection
+   object managed by the connection manager) should not be deleted
+   manually.
 
    The underlying reason is that all C++ objects are realized in the
    Extern C interface as mutable structs containing a void pointer to