Update tuning.md

http://stackoverflow.com/questions/9699071/what-is-the-javas-internal-represention-for-string-modified-utf-8-utf-16

Author: Andrew Ash <[email protected]>

Closes apache#384 from ash211/patch-2 and squashes the following commits:

da1b0be [Andrew Ash] Update tuning.md

docs/tuning.md

@@ -90,9 +90,10 @@ than the "raw" data inside their fields. This is due to several reasons:
 * Each distinct Java object has an "object header", which is about 16 bytes and contains information
   such as a pointer to its class. For an object with very little data in it (say one `Int` field), this
   can be bigger than the data.
-* Java Strings have about 40 bytes of overhead over the raw string data (since they store it in an
+* Java `String`s have about 40 bytes of overhead over the raw string data (since they store it in an
   array of `Char`s and keep extra data such as the length), and store each character
-  as *two* bytes due to Unicode. Thus a 10-character string can easily consume 60 bytes.
+  as *two* bytes due to `String`'s internal usage of UTF-16 encoding. Thus a 10-character string can
+  easily consume 60 bytes.
 * Common collection classes, such as `HashMap` and `LinkedList`, use linked data structures, where
   there is a "wrapper" object for each entry (e.g. `Map.Entry`). This object not only has a header,
   but also pointers (typically 8 bytes each) to the next object in the list.