doc: update stability index

This simplifies the stability index to 4 levels:

0 - deprecated
1 - experimental / feature-flagged
2 - stable
3 - locked

Domains has been downgraded to deprecated, assert has been
downgraded to stable. Timers and Module remain locked. All
other APIs are now stable.

PR-URL: https://github.com/iojs/io.js/pull/943
Fixes: https://github.com/iojs/io.js/issues/930
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>

doc/api/string_decoder.markdown

@@ -1,6 +1,6 @@
 # StringDecoder
 
-    Stability: 3 - Stable
+    Stability: 2 - Stable
 
 To use this module, do `require('string_decoder')`. StringDecoder decodes a
 buffer to a string. It is a simple interface to `buffer.toString()` but provides