[07 of 10]: docs: show log levels in bulleted list

The 'README.md' and crate-level API docs are both updated to show
the available logging levels in a bulleted list (rather then a
comma-separated list in the prose), and to note that they correspond
to the externally defined log::Level enum in the 'log' crate.

For the README.md file, this is an addition. The universe of valid
log levels was not previously listed there.

The order of the log levels is also now sorted in the list highest
precedence to lowest:
    error, warn, info, debug, trace

The previous order in which they were presented was:
    debug, error, info, warn, trace

Author: salewski

README.md

@@ -57,6 +57,17 @@ $ RUST_LOG=INFO ./main
 [2018-11-03T06:09:06Z INFO  default] starting up
 ```
 
+The log levels that may be specified correspond to the [`log::Level`][level-enum]
+enum from the `log` crate. They are:
+
+   * `error`
+   * `warn`
+   * `info`
+   * `debug`
+   * `trace`
+
+[level-enum]:  https://docs.rs/log/latest/log/enum.Level.html  "log::Level (docs.rs)"
+
 `env_logger` can be configured in other ways besides an environment variable. See [the examples](https://github.com/env-logger-rs/env_logger/tree/master/examples) for more approaches.
 
 ### In tests

src/lib.rs

@@ -110,10 +110,18 @@
 //! log level is optional. If omitted, all logging for the item (and its
 //! children) will be enabled.
 //!
-//! When specifying a logging level, the name provided must be one of the
-//! strings `debug`, `error`, `info`, `warn`, or `trace`. The letter case is
-//! not significant for the logging level names; e.g., `debug`, `DEBUG`, and
-//! `dEbuG` all represent the same logging level.
+//! The names of the log levels that may be specified correspond to the
+//! variations of the [`log::Level`][level-enum] enum from the `log`
+//! crate. They are:
+//!
+//!    * `error`
+//!    * `warn`
+//!    * `info`
+//!    * `debug`
+//!    * `trace`
+//!
+//! The letter case is not significant for the logging level names; e.g.,
+//! `debug`, `DEBUG`, and `dEbuG` all represent the same logging level.
 //!
 //! As the log level for a module is optional, the module to enable logging for
 //! is also optional. **If only a level is provided, then the global log
@@ -238,6 +246,7 @@
 //! ```
 //!
 //! [gh-repo-examples]: https://github.com/env-logger-rs/env_logger/tree/master/examples
+//! [level-enum]: https://docs.rs/log/latest/log/enum.Level.html
 //! [log-crate-url]: https://docs.rs/log/
 //! [`Builder`]: struct.Builder.html
 //! [`Builder::is_test`]: struct.Builder.html#method.is_test