Add docstring for sandboxed

Author: Vinh Tran

rust/private/rust.bzl

@@ -609,6 +609,13 @@ _common_attrs = {
     "_error_format": attr.label(
         default = Label("//:error_format"),
     ),
+    "_experimental_toolchain_generated_sysroot": attr.label(
+        default = Label("//rust/settings:experimental_toolchain_generated_sysroot"),
+        doc = (
+            "Label to a boolean build setting that lets the rule knows wheter to set --sysroot to rustc" +
+            "This flag is only relevant when used together with --@rules_rust//rust/settings:experimental_toolchain_generated_sysroot."
+        ),
+    ),
     "_extra_exec_rustc_flag": attr.label(
         default = Label("//:extra_exec_rustc_flag"),
     ),
@@ -634,9 +641,6 @@ _common_attrs = {
     "_per_crate_rustc_flag": attr.label(
         default = Label("//:experimental_per_crate_rustc_flag"),
     ),
-    "_experimental_toolchain_generated_sysroot": attr.label(
-        default = Label("@rules_rust//rust/settings:experimental_toolchain_generated_sysroot"),
-    ),
     "_process_wrapper": attr.label(
         doc = "A process wrapper for running rustc on all platforms.",
         default = Label("//util/process_wrapper"),

rust/private/rustc.bzl

@@ -820,6 +820,7 @@ def construct_arguments(
         build_metadata (bool): Generate CLI arguments for building *only* .rmeta files. This requires use_json_output.
         force_depend_on_objects (bool): Force using `.rlib` object files instead of metadata (`.rmeta`) files even if they are available.
         skip_expanding_rustc_env (bool): Whether to skip expanding CrateInfo.rustc_env_attr
+        sandboxed (bool): Whether the action using the constructed arguments is sandboxed
 
     Returns:
         tuple: A tuple of the following items

rust/toolchain.bzl

@@ -797,13 +797,6 @@ rust_toolchain = rule(
                 "This flag is only relevant when used together with --@rules_rust//rust/settings:experimental_use_global_allocator."
             ),
         ),
-        "_experimental_toolchain_generated_sysroot": attr.label(
-            default = Label("//rust/settings:experimental_toolchain_generated_sysroot"),
-            doc = (
-                "Label to a boolean build setting that lets the rule knows wheter to set --sysroot to rustc" +
-                "This flag is only relevant when used together with --@rules_rust//rust/settings:experimental_toolchain_generated_sysroot."
-            ),
-        ),
         "_no_std": attr.label(
             default = Label("//:no_std"),
         ),