add docs on task migration (#50047)

(cherry picked from commit ff23b37)

Author: IanButterworth

base/threadingconstructs.jl

@@ -8,6 +8,25 @@ export threadid, nthreads, @threads, @spawn,
 
 Get the ID number of the current thread of execution. The master thread has
 ID `1`.
+
+# Examples
+```julia-repl
+julia> Threads.threadid()
+1
+
+julia> Threads.@threads for i in 1:4
+          println(Threads.threadid())
+       end
+4
+2
+5
+4
+```
+
+!!! note
+    The thread that a task runs on may change if the task yields, which is known as [`Task Migration`](@ref man-task-migration).
+    For this reason in most cases it is not safe to use `threadid()` to index into, say, a vector of buffer or stateful objects.
+
 """
 threadid() = Int(ccall(:jl_threadid, Int16, ())+1)
 
@@ -220,7 +239,7 @@ For example, the above conditions imply that:
 - Write only to locations not shared across iterations (unless a lock or atomic operation is
   used).
 - The value of [`threadid()`](@ref Threads.threadid) may change even within a single
-  iteration.
+  iteration. See [`Task Migration`](@ref man-task-migration)
 
 ## Schedulers
 
@@ -346,8 +365,10 @@ the _value_ of a variable, isolating the asynchronous code from changes to
 the variable's value in the current task.
 
 !!! note
-    See the manual chapter on [multi-threading](@ref man-multithreading)
-    for important caveats. See also the chapter on [threadpools](@ref man-threadpools).
+    The thread that the task runs on may change if the task yields, therefore `threadid()` should not
+    be treated as constant for a task. See [`Task Migration`](@ref man-task-migration), and the broader
+    [multi-threading](@ref man-multithreading) manual for further important caveats.
+    See also the chapter on [threadpools](@ref man-threadpools).
 
 !!! compat "Julia 1.3"
     This macro is available as of Julia 1.3.

doc/src/manual/multi-threading.md

@@ -372,6 +372,18 @@ threads in Julia:
     This may require some transitional work across the ecosystem before threading
     can be widely adopted with confidence. See the next section for further details.
 
+## [Task Migration](@id man-task-migration)
+
+After a task starts running on a certain thread (e.g. via [`@spawn`](@ref Threads.@spawn) or
+[`@threads`](@ref Threads.@threads)), it may move to a different thread if the task yields.
+
+This means that [`threadid()`](@ref Threads.threadid) should not be treated as constant within a task, and therefore
+should not be used to index into a vector of buffers or stateful objects.
+
+!!! compat "Julia 1.7"
+    Task migration was introduced in Julia 1.7. Before this tasks always remained on the same thread that they were
+    started on.
+
 ## Safe use of Finalizers
 
 Because finalizers can interrupt any code, they must be very careful in how