Remove ^ from Cargo.toml + small improvements for migration.md

Cargo.toml

@@ -19,16 +19,16 @@ travis-ci = { repository = "PyO3/pyo3", branch = "master" }
 appveyor = { repository = "fafhrd91/pyo3" }
 
 [dependencies]
-indoc = "^0.3.4"
-inventory = "^0.1.4"
-libc = "^0.2.62"
+indoc = "0.3.4"
+inventory = "0.1.4"
+libc = "0.2.62"
 num-bigint = { version = "0.2", optional = true }
 num-complex = { version = "0.2", optional = true }
-num-traits = "^0.2.8"
+num-traits = "0.2.8"
 parking_lot = { version = "0.10", features = ["nightly"] }
-paste = "^0.1.6"
+paste = "0.1.6"
 pyo3cls = { path = "pyo3cls", version = "=0.9.0-alpha.1" }
-unindent = "^0.1.4"
+unindent = "0.1.4"
 
 [dev-dependencies]
 assert_approx_eq = "1.1.0"

guide/src/migration.md

@@ -40,9 +40,10 @@ For more, see [the constructor section](https://pyo3.rs/master/class.html#constr
 
 ### PyCell
 PyO3 0.9 introduces [`PyCell`](https://pyo3.rs/master/doc/pyo3/pycell/struct.PyCell.html), which is
-a [`RefCell`](https://doc.rust-lang.org/std/cell/struct.RefCell.html) like object wrapper
-for dynamically ensuring
-[The Rules of References](https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#the-rules-of-references).
+a [`RefCell`](https://doc.rust-lang.org/std/cell/struct.RefCell.html)-like object wrapper
+for ensuring Rust's rules regarding aliasing of references are upheld.
+For more detail, see the
+[Rust Book's section on Rust's rules of references](https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#the-rules-of-references)
 
 For `#[pymethods]` or `#[pyfunction]`s, your existing code should continue to work without any change.
 Python exceptions will automatically be raised when your functions are used in a way which breaks Rust's