Add note about running clean before using -vv

This solves an issue that came up in the beginners chat that took about fifteen minutes to finally understand what was going on.

This should help users who are actively developing `build.rs` consistently get debugging information printed to the terminal.

src/doc/build-script.md

@@ -47,9 +47,13 @@ the source directory of the build script’s package.
 All the lines printed to stdout by a build script are written to a file like
 `target/debug/build/<pkg>/output` (the precise location may depend on your
 configuration). If you would like to see such output directly in your terminal,
-invoke cargo as 'very verbose' with the `-vv` flag. Any line that starts with
-`cargo:` is interpreted directly by Cargo. This line must be of the form
-`cargo:key=value`, like the examples below:
+invoke cargo as 'very verbose' with the `-vv` flag. Note that if neither the
+build script nor project source files are modified, subsequent calls to
+cargo with the `-vv` will **not** print output to the terminal because a
+new build is not executed. Run `cargo clean` before each cargo invokation
+if you want to ensure that output is always displayed on your terminal.
+Any line that starts with `cargo:` is interpreted directly by Cargo. 
+This line must be of the form `cargo:key=value`, like the examples below:
 
 ```notrust
 # specially recognized by Cargo