Document the use of rev to depend on pull requests

Author: dtolnay

src/doc/src/reference/specifying-dependencies.md

@@ -147,6 +147,13 @@ the latest commit on a branch named `next`:
 regex = { git = "https://github.com/rust-lang/regex", branch = "next" }
 ```
 
+Anything that is not a branch or tag falls under `rev`. This can be a commit
+hash like `rev = "4c59b707"`, or a named reference exposed by the remote
+repository such as `rev = "refs/pull/493/head"`. What references are available
+varies by where the repo is hosted; GitHub in particular exposes a reference to
+the most recent commit of every pull request as shown, but other git hosts often
+provide something equivalent, possibly under a different naming scheme.
+
 Once a `git` dependency has been added, Cargo will lock that dependency to the
 latest commit at the time. New commits will not be pulled down automatically
 once the lock is in place. However, they can be pulled down manually with