Edit docs for rustc_middle::dep_graph::dep_node

Fixes some missing punctuation, and adds a blank line
for a more concise summary.

src/librustc_middle/dep_graph/dep_node.rs

@@ -1,7 +1,9 @@
 //! This module defines the `DepNode` type which the compiler uses to represent
-//! nodes in the dependency graph. A `DepNode` consists of a `DepKind` (which
+//! nodes in the dependency graph.
+//!
+//! A `DepNode` consists of a `DepKind` (which
 //! specifies the kind of thing it represents, like a piece of HIR, MIR, etc)
-//! and a `Fingerprint`, a 128 bit hash value the exact meaning of which
+//! and a `Fingerprint`, a 128-bit hash value the exact meaning of which
 //! depends on the node's `DepKind`. Together, the kind and the fingerprint
 //! fully identify a dependency node, even across multiple compilation sessions.
 //! In other words, the value of the fingerprint does not depend on anything
@@ -11,9 +13,9 @@
 //! uniquely identify a given commit and has a few advantages:
 //!
 //! * A `DepNode` can simply be serialized to disk and loaded in another session
-//!   without the need to do any "rebasing (like we have to do for Spans and
-//!   NodeIds) or "retracing" like we had to do for `DefId` in earlier
-//!   implementations of the dependency graph.
+//!   without the need to do any "rebasing" (like we have to do for Spans and
+//!   NodeIds) or "retracing" (like we had to do for `DefId` in earlier
+//!   implementations of the dependency graph).
 //! * A `Fingerprint` is just a bunch of bits, which allows `DepNode` to
 //!   implement `Copy`, `Sync`, `Send`, `Freeze`, etc.
 //! * Since we just have a bit pattern, `DepNode` can be mapped from disk into
@@ -42,7 +44,7 @@
 //!   `DefId` it was computed from. In other cases, too much information gets
 //!   lost during fingerprint computation.
 //!
-//! The `DepConstructor` enum, together with `DepNode::new()` ensures that only
+//! The `DepConstructor` enum, together with `DepNode::new()`, ensures that only
 //! valid `DepNode` instances can be constructed. For example, the API does not
 //! allow for constructing parameterless `DepNode`s with anything other
 //! than a zeroed out fingerprint. More generally speaking, it relieves the