doc: update assert.doesNotThrow() docs

It takes an optional "expected exception" argument that is not used meaningfully
but is nevertheless documented. Undocument it, it confuses casual readers of the
documentation.

Fixes #3935.

doc/api/assert.markdown

@@ -74,7 +74,7 @@ Custom error validation:
       "unexpected error"
     );
 
-## assert.doesNotThrow(block, [error], [message])
+## assert.doesNotThrow(block, [message])
 
 Expects `block` not to throw an error, see assert.throws for details.
 

lib/assert.js

@@ -327,7 +327,7 @@ assert.throws = function(block, /*optional*/error, /*optional*/message) {
 };
 
 // EXTENSION! This is annoying to write outside this module.
-assert.doesNotThrow = function(block, /*optional*/error, /*optional*/message) {
+assert.doesNotThrow = function(block, /*optional*/message) {
   _throws.apply(this, [false].concat(pSlice.call(arguments)));
 };