message: [pdsl_core] Add documentation to env module
diff: +//! Contract environments.
  +//!
  +//! A contract is able to operate on different environments.
  +//!
  +//! Currently the SRML environment operating directly on the
  +//! substrate runtime module library (SRML) and the test
  +//! environment for testing and inspecting contracts are
  +//! provided.
  +//!
  +//! By default the SRML environment is used.
  +//! To enable the test environment the `test-env` crate feature
  +//! has to be enabled.
  +
  #[cfg(not(feature = "test-env"))]
  mod srml_env;
  