Rollup merge of rust-lang#21314 - fenhl:patch-1, r=steveklbanik

See [this document](https://gist.github.com/0xabad1dea/8870b192fd1758743f66) by @0xabad1dea for the rationale.

Author: barosl

src/libserialize/json.rs

@@ -34,7 +34,7 @@
 //!
 //! An object is a series of string keys mapping to values, in `"key": value` format.
 //! Arrays are enclosed in square brackets ([ ... ]) and objects in curly brackets ({ ... }).
-//! A simple JSON document encoding a person, his/her age, address and phone numbers could look like
+//! A simple JSON document encoding a person, their age, address and phone numbers could look like
 //!
 //! ```ignore
 //! {