Link the memory allocation guide from the MM docs

I just went looking for the memory allocation guide in the MM docs instead
of in the core API. For the benefit of the next person who makes that
mistake, link to it from the MM docs.

Signed-off-by: Matthew Wilcox <[e-mail]>
Acked-by: Mike Rapoport <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/memory-allocation.rst

···
+.. _memory-allocation:
+
 =======================
 Memory Allocation Guide
 =======================

Documentation/vm/index.rst

···
 Linux Memory Management Documentation
 =====================================
 
-This is a collection of documents about Linux memory management (mm) subsystem.
+This is a collection of documents about the Linux memory management (mm)
+subsystem.  If you are looking for advice on simply allocating memory,
+see the :ref:`memory-allocation`.
 
 User guides for MM features
 ===========================