Move "upgrade mdbook" description to the build-from-source section

guide/src/guide/installation.md

@@ -30,6 +30,9 @@ cargo install mdbook
 
 This will automatically download mdBook from [crates.io], build it, and install it in Cargo's global binary directory (`~/.cargo/bin/` by default).
 
+You can run `cargo install mdbook` again whenever you want to update to a new version.
+That command will check if there is a newer version, and re-install mdBook if a newer version is found.
+
 To uninstall, run the command `cargo uninstall mdbook`.
 
 [Rust installation page]: https://www.rust-lang.org/tools/install
@@ -47,16 +50,6 @@ cargo install --git https://github.com/rust-lang/mdBook.git mdbook
 
 Again, make sure to add the Cargo bin directory to your `PATH`.
 
-
-### Updating mdbook from source
-
-```sh
-cargo install mdbook
-```
-
-This will automatically download mdBook from [crates.io], build it, and re-install it in Cargo's global binary directory (`~/.cargo/bin/` by default).
-
-
 ## Own modifications and contributing
 
 If you are interested in making modifications to mdBook itself, check out the [Contributing Guide] for more information.