Clarify cargo fallback behavior for rustup link (rust-lang#1273)

Author: couchand

src/building/how-to-build-and-run.md

@@ -233,6 +233,12 @@ The rustup toolchain points to the specified toolchain compiled in your `build`
 so the rustup toolchain will be updated whenever `x.py build` or `x.py test` are run for
 that toolchain/stage.
 
+**Note:** the toolchain we've built does not include `cargo`.  In this case, `rustup` will
+fall back to using `cargo` from the installed `nightly`, `beta`, or `stable` toolchain
+(in that order).  If you need to use unstable `cargo` flags, be sure to run
+`rustup install nightly` if you haven't already.  See the
+[rustup documentation on custom toolchains](https://rust-lang.github.io/rustup/concepts/toolchains.html#custom-toolchains).
+
 ## Other `x.py` commands
 
 Here are a few other useful `x.py` commands. We'll cover some of them in detail