doc: Update description of logging spec and ::rt module

Author: brson

doc/rust.md

@@ -3268,17 +3268,26 @@ The runtime contains a system for directing [logging
 expressions](#log-expressions) to a logging console and/or internal logging
 buffers. Logging expressions can be enabled per module.
 
-Logging output is enabled by setting the `RUST_LOG` environment variable.
-`RUST_LOG` accepts a logging specification that is a comma-separated list of
-paths. For each module containing log expressions, if `RUST_LOG` contains the
-path to that module or a parent of that module, then its logs will be output
-to the console. The path to an module consists of the crate name, any parent
-modules, then the module itself, all separated by double colons (`::`).
+Logging output is enabled by setting the `RUST_LOG` environment
+variable.  `RUST_LOG` accepts a logging specification made up of a
+comma-separated list of paths, with optional log levels. For each
+module containing log expressions, if `RUST_LOG` contains the path to
+that module or a parent of that module, then logs of the appropriate
+level will be output to the console.
+
+The path to a module consists of the crate name, any parent modules,
+then the module itself, all separated by double colons (`::`).  The
+optional log level can be appended to the module path with an equals
+sign (`=`) followed by the log level, from 0 to 3, inclusive. Level 0
+is the error level, 1 is warning, 2 info, and 3 debug. Any logs
+less than or equal to the specified level will be output. If not
+specified then log level 3 is assumed.
 
 As an example, to see all the logs generated by the compiler, you would set
 `RUST_LOG` to `rustc`, which is the crate name (as specified in its `link`
 [attribute](#attributes)). To narrow down the logs to just crate resolution,
-you would set it to `rustc::metadata::creader`.
+you would set it to `rustc::metadata::creader`. To see just error logging
+use `rustc=0`.
 
 Note that when compiling either `.rs` or `.rc` files that don't specifiy a
 crate name the crate is given a default name that matches the source file,
@@ -3305,7 +3314,7 @@ of runtime logging modules follows.
 * `::rt::gc` Garbage collection
 * `::rt::stdlib` Functions used directly by the standard library
 * `::rt::kern` The runtime kernel
-* `::rt::backtrace` Unused
+* `::rt::backtrace` Log a backtrace on task failure
 * `::rt::callback` Unused