rust/cargo: add support for a custom target directory

This can avoid having to wait for ALE or ALE being blocked on other
cargo actions within the same crate.

doc/ale-rust.txt

@@ -174,6 +174,18 @@ g:ale_rust_cargo_clippy_options
   only `cargo clippy` supports (e.g. `--deny`).
 
 
+g:ale_rust_cargo_target_dir
+                                                  *g:ale_rust_cargo_target_dir*
+                                                  *b:ale_rust_cargo_target_dir*
+
+  Type: |String|
+  Default: `''`
+
+  Use a custom target directory when running the commands for ALE. This can
+  help to avoid "waiting for file lock on build directory" messages when
+  running `cargo` commands manually while ALE is performing its checks.
+
+
 ===============================================================================
 rls                                                              *ale-rust-rls*