Add a remark about Sync and Send traits requirement for resources

book/en/src/by-example/resources.md

@@ -30,6 +30,11 @@ task.
 Thus, a task `#[local]` resource can only be accessed by one singular task.
 Attempting to assign the same `#[local]` resource to more than one task is a compile-time error.
 
+Types of `#[local]` resources must implement [`Send`] trait as they are being sent from `init`
+to target task and thus crossing the thread boundary.
+
+[`Send`]: https://doc.rust-lang.org/stable/core/marker/trait.Send.html
+
 The example application shown below contains two tasks where each task has access to its own
 `#[local]` resource, plus that the `idle` task has its own `#[local]` as well.
 
@@ -51,6 +56,11 @@ A special use-case of local resources are the ones specified directly in the res
 initialized in `#[init]`.
 Moreover, local resources in `#[init]` and `#[idle]` have `'static` lifetimes, this is safe since both are not re-entrant.
 
+Types of `#[task(local = [..])]` resources have to be neither [`Send`] nor [`Sync`] as they
+are not crossing any thread boundary.
+
+[`Sync`]: https://doc.rust-lang.org/stable/core/marker/trait.Sync.html
+
 In the example below the different uses and lifetimes are shown:
 
 ``` rust
@@ -95,6 +105,8 @@ $ cargo run --target thumbv7m-none-eabi --example lock
 {{#include ../../../../ci/expected/lock.run}}
 ```
 
+Types of `#[shared]` resources have to be both [`Send`] and [`Sync`].
+
 ## Multi-lock
 
 As an extension to `lock`, and to reduce rightward drift, locks can be taken as tuples. The