doc: add a link to constance::sync::Mutex

Author: yvt

src/constance/src/lib.md

@@ -203,7 +203,7 @@ fn task_body(_: usize) {
 [`KernelCfg2`]: crate::kernel::KernelCfg2
 [`Task`]: crate::kernel::Task
 
-Configuration functions are highly composable as they can call other configuration functions in turn. In some sense, this is a way to attribute a certain semantics to a group of kernel objects, making them behave in a meaningful way as a whole, and expose a whole new, higher-level interface. For example, a mutex object similar to `std::sync::Mutex` can be created by combining a low-level mutex object (not implemented yet) and a [`Hunk`]`<System, UnsafeCell<T>>`.
+Configuration functions are highly composable as they can call other configuration functions in turn. In some sense, this is a way to attribute a certain semantics to a group of kernel objects, making them behave in a meaningful way as a whole, and expose a whole new, higher-level interface. For example, a [mutex object] similar to `std::sync::Mutex` can be created by combining a low-level mutex object (not implemented yet) and a [`Hunk`]`<System, UnsafeCell<T>>`.
 
 ```rust
 # #![feature(const_fn)]
@@ -230,6 +230,7 @@ mod m {
 ```
 
 [`Hunk`]: crate::kernel::Hunk
+[mutex object]: crate::sync::Mutex
 
 The constructors of kernel objects are configuration functions by themselves, but they are different from normal configuration functions in that they can actually mutate the contents of `CfgBuilder` (which `build!` will use to create kernel structures in the final form), ultimately shaping the outcome of the configuration process. Therefore, they are the smallest building blocks of configuration functions.