pw_thread_embos: add thread creation for embOS

Adds thread creation support to embOS.

Also updates existing documentation to reflect that this is now
supported.

Change-Id: Id95dbb5f7b283733b7cd071831ddcf95c0c549d0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44144
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
diff --git a/pw_thread/docs.rst b/pw_thread/docs.rst
index 99003cd..9b03369 100644
--- a/pw_thread/docs.rst
+++ b/pw_thread/docs.rst
@@ -58,7 +58,7 @@
   * - ThreadX
     - :ref:`module-pw_thread_threadx`
   * - embOS
-    - Planned
+    - :ref:`module-pw_thread_embos`
   * - STL
     - :ref:`module-pw_thread_stl`
   * - Zephyr