Clarify that [replace] requires a version number.

Closes rust-lang#4212

src/doc/src/reference/manifest.md

@@ -1000,11 +1000,11 @@ other copies. The syntax is similar to the `[dependencies]` section:
 
 Each key in the `[replace]` table is a [package ID
 specification](pkgid-spec.md), which allows arbitrarily choosing a node in the
-dependency graph to override. The value of each key is the same as the
-`[dependencies]` syntax for specifying dependencies, except that you can't
-specify features. Note that when a crate is overridden the copy it's overridden
-with must have both the same name and version, but it can come from a different
-source (e.g., git or a local path).
+dependency graph to override (the 3-part version number is required). The
+value of each key is the same as the `[dependencies]` syntax for specifying
+dependencies, except that you can't specify features. Note that when a crate
+is overridden the copy it's overridden with must have both the same name and
+version, but it can come from a different source (e.g., git or a local path).
 
 More information about overriding dependencies can be found in the [overriding
 dependencies][replace] section of the documentation.