Bug 1372743: Add updating Cargo.lock file in 'toolkit/library/rust' using 'cargo update' as step in the docs to add a rust crate. r=rillian

MozReview-Commit-ID: KDasHNTCKrq

--HG--
extra : rebase_source : dfb7be3ff99ee1316c8fb19ca9d4456731f05e4a

build/docs/rust.rst

@@ -14,7 +14,10 @@ Linking Rust Crates into libxul
 
 Rust crates that you want to link into libxul should be listed in the
 ``dependencies`` section of `toolkit/library/rust/shared/Cargo.toml <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/Cargo.toml>`_.
-You'll also need to add an ``extern crate`` reference to `toolkit/library/rust/shared/lib.rs <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_.
+After adding your crate, execute ``cargo update -p gkrust-shared`` in
+``toolkit/library/rust`` to update the Cargo.lock file.  You'll also
+need to add an ``extern crate`` reference to
+`toolkit/library/rust/shared/lib.rs <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_.
 This ensures that the Rust code will be linked properly into libxul as well
 as the copy of libxul used for gtests.