Add env attr to docs for proto_plugin

docs/custom_plugins.rst

@@ -124,6 +124,11 @@ API Reference
      - false
      - ``[]``
      - A list of extra command line arguments to pass directly to protoc, not as plugin options
+   * - ``extra_protoc_args``
+     - ``string_dict``
+     - false
+     - ``{}``
+     - A dictionary of key-value environment variables to use when invoking protoc for this plugin. Must be empty if use_built_in_shell_environment is true
    * - ``exclusions``
      - ``string_list``
      - false

internal/plugin.bzl

@@ -51,7 +51,7 @@ proto_plugin = rule(
             default = False,
         ),
         "env": attr.string_dict(
-            doc = "Sets the dictionary of environment variables to use when invoking protoc. Must be None if use_built_in_shell_environment is true.",
+            doc = "A dictionary of key-value environment variables to use when invoking protoc for this plugin. Must be empty if use_built_in_shell_environment is true",
             default = {},
         ),
         "extra_protoc_args": attr.string_list(