Modify the crate docs for bail!

The docs currently say that the bail! macro is like using try! or ?.
This isn't really correct. `try!` and `?` operate on a `Result` type,
returning the `Ok` value, if it is `Ok`, or returning early from the
function if it is an `Err`. `bail!` is equivalent to just returning
an `Err`, period. This change makes that a little more clear.

Author: dead10ck

src/lib.rs

@@ -231,9 +231,10 @@
 //! ## The `bail!` macro
 //!
 //! The above method of introducing new errors works but is a little
-//! verbose. Instead we can use the `bail!` macro, which, much like `try!`
-//! and `?`, performs an early return with conversions. With `bail!` the
-//! previous examples look like:
+//! verbose. Instead, we can use the `bail!` macro, which performs an early return
+//! with conversions done automatically.
+//!
+//! With `bail!` the previous examples look like:
 //!
 //! ```
 //! # #[macro_use] extern crate error_chain;