Clarify /rules/lib/builtins/repository_ctx description of default wor…

…king_directory for execute()

Fixes bazelbuild#21897 per feedback from @meteorcloudy.

src/main/java/com/google/devtools/build/lib/bazel/repository/starlark/StarlarkBaseExternalContext.java

@@ -1561,7 +1561,8 @@ private static String profileArgsDesc(String method, List<String> args) {
             named = true,
             doc =
                 "Working directory for command execution.\n"
-                    + "Can be relative to the repository root or absolute."),
+                    + "Can be relative to the repository root or absolute.\n"
+                    + "The default is the repository root."),
       })
   public StarlarkExecutionResult execute(
       Sequence<?> arguments, // <String> or <StarlarkPath> or <Label> expected