Correct mistake about supporting sub-makes and document CARGO_MAKEFLAGS

Sub-makes are currently not supported as the jobserver crate only
sets CARGO_MAKEFLAGS which GNU Make doesn't understand.

This might be reasonable as cargo overriding existing MAKEFLAGS
variables would be weird, but also makes the statement in the
reference false.

It's helpful for build script authors who want to invoke make
to know how to get make subprocesses work with parallel builds.

Author: est31

src/doc/src/reference/environment-variables.md

@@ -253,6 +253,15 @@ let out_dir = env::var("OUT_DIR").unwrap();
                          current working directory of the build script when it
                          starts.
 * `CARGO_MANIFEST_LINKS` — the manifest `links` value.
+* `CARGO_MAKEFLAGS` — Contains parameters needed for Cargo's [jobserver]
+                      implementation to parallelize subprocesses.
+                      Rustc or cargo invocations from build.rs can already
+                      read `CARGO_MAKEFLAGS`, but GNU Make requires the
+                      flags to be specified either directly as arguments,
+                      or through the `MAKEFLAGS` environment variable.
+                      Currently Cargo doesn't set the `MAKEFLAGS` variable,
+                      but it's free for build scripts invoking GNU Make
+                      to set it to the contents of `CARGO_MAKEFLAGS`.
 * `CARGO_FEATURE_<name>` — For each activated feature of the package being
                            built, this environment variable will be present
                            where `<name>` is the name of the feature uppercased
@@ -288,10 +297,9 @@ let out_dir = env::var("OUT_DIR").unwrap();
                that care should be taken when interpreting this environment
                variable. For historical purposes this is still provided but
                recent versions of Cargo, for example, do not need to run `make
-               -j` as it'll automatically happen. Cargo implements its own
-               [jobserver] and will allow build scripts to inherit this
-               information, so programs compatible with GNU make jobservers will
-               already have appropriately configured parallelism.
+               -j`, and instead can set the `MAKEFLAGS` env var to the content
+               of `CARGO_MAKEFLAGS` to activate the use of Cargo's GNU Make
+               compatible [jobserver] for sub-make invocations.
 * `OPT_LEVEL`, `DEBUG` — values of the corresponding variables for the
                          profile currently being built.
 * `PROFILE` — `release` for release builds, `debug` for other builds.