[docs] Clarify async proxying behavior and add link to asyncify docs. (emscripten-core#26150)

Author: dschuff

site/source/docs/porting/asyncify.rst

@@ -161,6 +161,8 @@ You will see something like this:
 That shows that the C code only continued to execute after the async JS
 completed.
 
+.. _marking_async_functions:
+
 Marking JS library functions as async
 #####################################
 

site/source/docs/porting/pthreads.rst

@@ -59,8 +59,11 @@ can also be used directly for more precise control over how work is run on
 different threads.
 
 When a JS library function is marked as both ``__proxy: 'sync'`` and ``__async:
-'auto'`` the calling thread will block until the async operation on the
-main thread is completed (i.e. the returned promise is resolved).  This allows
+'auto'``, it is expected to return a promise
+(see :ref:`marking_async_functions`).
+The calling thread will block until the async operation on the
+main thread is completed (i.e. the returned promise is resolved), and the value
+that the promise resolves to will be returned to the caller. This allows
 functions that would require :ref:`ASYNCIFY` (when called from the main browser
 thread) to be called without :ref:`ASYNCIFY` from a background thread.  In other
 words, background threads can block on proxied work even without :ref:`ASYNCIFY`