Reword E0626 to mention static coroutine

compiler/rustc_error_codes/src/error_codes/E0626.md

@@ -1,4 +1,4 @@
-This error occurs because a borrow in a coroutine persists across a
+This error occurs because a borrow in a movable coroutine persists across a
 yield point.
 
 Erroneous code example:
@@ -15,19 +15,35 @@ let mut b = #[coroutine] || {
 Pin::new(&mut b).resume(());
 ```
 
-At present, it is not permitted to have a yield that occurs while a
+Coroutines may be either unmarked, or marked with `static`. If it is unmarked,
-borrow is still in scope. To resolve this error, the borrow must
+then the coroutine is considered "movable". At present, it is not permitted to
-either be "contained" to a smaller scope that does not overlap the
+have a yield in a movable coroutine that occurs while a borrow is still in
-yield or else eliminated in another way. So, for example, we might
+scope. To resolve this error, the coroutine may be marked `static`:
-resolve the previous example by removing the borrow and just storing
+
-the integer by value:
+```
+# #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
+# use std::ops::Coroutine;
+# use std::pin::Pin;
+let mut b = #[coroutine] static || { // <-- note the static keyword
+    let a = &String::from("hello, world");
+    yield ();
+    println!("{}", a);
+};
+let mut b = std::pin::pin!(b);
+b.as_mut().resume(());
+```
+
+If the coroutine must remain movable, for example to be used as `Unpin`
+without pinning it on the stack or in an allocation, we can alternatively
+resolve the previous example by removing the borrow and just storing the
+type by value:
 
 ```
 # #![feature(coroutines, coroutine_trait, stmt_expr_attributes)]
 # use std::ops::Coroutine;
 # use std::pin::Pin;
 let mut b = #[coroutine] || {
-    let a = 3;
+    let a = String::from("hello, world");
     yield ();
     println!("{}", a);
 };