Fix explanation of Cargo's behavior

https://github.com/rust-lang/rust/pull/25080/files#r29634986

Author: steveklabnik

src/doc/trpl/guessing-game.md

@@ -410,24 +410,29 @@ $ cargo build
    Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
 ```
 
-So, we told Cargo we wanted any version of `rand`, and so it fetched the
-latest version at the time this was written, `v0.3.8`. But what happens
-when next week, version `v0.4.0` comes out, which changes something with
-`rand`, and it includes a breaking change? After all, a `v0.y.z` version
-in SemVer can change every release.
+So, we told Cargo we wanted any version of `rand`, and so it fetched the latest
+version at the time this was written, `v0.3.8`. But what happens when next
+week, version `v0.3.9` comes out, with an important bugfix? While getting
+bugfixes is important, what if `0.3.9` contains a regression that breaks our
+code?
 
 The answer to this problem is the `Cargo.lock` file you’ll now find in your
 project directory. When you build your project for the first time, Cargo
 figures out all of the versions that fit your criteria, and then writes them
 to the `Cargo.lock` file. When you build your project in the future, Cargo
 will see that the `Cargo.lock` file exists, and then use that specific version
 rather than do all the work of figuring out versions again. This lets you
-have a repeatable build automatically.
+have a repeatable build automatically. In other words, we’ll stay at `0.3.8`
+until we explicitly upgrade, and so will anyone who we share our code with,
+thanks to the lock file.
 
-What about when we _do_ want to use `v0.4.0`? Cargo has another command,
+What about when we _do_ want to use `v0.3.9`? Cargo has another command,
 `update`, which says ‘ignore the lock, figure out all the latest versions that
 fit what we’ve specified. If that works, write those versions out to the lock
-file’.
+file’. But, by default, Cargo will only look for versions larger than `0.3.0`
+and smaller than `0.4.0`. If we want to move to `0.4.x`, we’d have to update
+the `Cargo.toml` directly. When we do, the next time we `cargo build`, Cargo
+will update the index and re-evaluate our `rand` requirements.
 
 There’s a lot more to say about [Cargo][doccargo] and [its
 ecosystem][doccratesio], but for now, that’s all we need to know. Cargo makes