[readme] improve t.throws/t.doesNotThrow documentation

* Described method behavior when "expected" parameter is a string.

* `throws`: Described method behavior when "expected" parameter is an object (i.e. validation object).

* Added usage example for when expected parameter is a regex.

Fixes #540.

readme.markdown

@@ -286,12 +286,15 @@ Aliases: `t.notLooseEqual()`, `t.notLooseEquals()`
 
 ## t.throws(fn, expected, msg)
 
-Assert that the function call `fn()` throws an exception. `expected`, if present, must be a `RegExp` or `Function`. The `RegExp` matches the string representation of the exception, as generated by `err.toString()`. The `Function` is the exception thrown (e.g. `Error`). `msg` is an optional description of the assertion.
+Assert that the function call `fn()` throws an exception. `expected`, if present, must be a `RegExp`, `Function`, or `Object`. The `RegExp` matches the string representation of the exception, as generated by `err.toString()`. For example, if you set `expected` to `/user/`, the test will pass only if the string representation of the exception contains the word `user`. Any other exception will result in a failed test. The `Function` is the exception thrown (e.g. `Error`). `Object` in this case corresponds to a so-called validation object, in which each property is tested for strict deep equality. This is very similar to how Node's `assert.throws()` method tests validation objects (please see the [Node _assert.throws()_ documentation](https://nodejs.org/api/assert.html#assert_assert_throws_fn_error_message) for an example). If `expected` is not of type `RegExp`, `Function`, or `Object`, or omitted entirely, any exception will result in a passed test. `msg` is an optional description of the assertion.
+
+Please note that the second parameter, `expected`, cannot be of type `string`. If a value of type `string` is provided for `expected`, then `t.throws(fn, expected, msg)`  will execute, but the value of `expected` will be set to  `undefined`, and the specified string will be set as the value for the `msg` parameter (regardless of what _actually_ passed as the third parameter). This can cause unexpected results, so please be mindful.
 
 ## t.doesNotThrow(fn, expected, msg)
 
-Assert that the function call `fn()` does not throw an exception. `expected`, if present, limits what should not be thrown. For example, set `expected` to `/user/` to fail the test only if the string representation of the exception contains the word `user`. Any other exception would pass the test. If `expected` is omitted, any exception will fail the test. `msg` is an optional description of the assertion.
+Assert that the function call `fn()` does not throw an exception. `expected`, if present, limits what should not be thrown, and must be a `RegExp` or `Function`. The `RegExp` matches the string representation of the exception, as generated by `err.toString()`. For example, if you set `expected` to `/user/`, the test will fail  only if the string representation of the exception contains the word `user`. Any other exception will result in a passed test.  The `Function` is the exception thrown (e.g. `Error`).  If `expected` is not of type `RegExp` or `Function`, or omitted entirely, any exception will result in a failed test. `msg` is an optional description of the assertion.
 
+Please note that the second parameter, `expected`, cannot be of type `string`. If a value of type `string` is provided for `expected`, then `t.doesNotThrows(fn, expected, msg)`  will execute, but the value of `expected` will be set to  `undefined`, and the specified string will be set as the value for the `msg` parameter (regardless of what _actually_ passed as the third parameter). This can cause unexpected results, so please be mindful.
 ## t.test(name, [opts], cb)
 
 Create a subtest with a new test handle `st` from `cb(st)` inside the current test `t`. `cb(st)` will only fire when `t` finishes. Additional tests queued up after `t` will not be run until all subtests finish.