[Docs] Fix PAL description in glossary

Documentation/glossary.rst

@@ -6,9 +6,20 @@ Glossary
 .. glossary::
 
    PAL
-      Platform Adaption Layer
+      Platform Adaptation Layer
 
-      .. todo:: Is this really correct?
+      PAL is the layer of Graphene that implements a narrow Drawbridge-like ABI
+      interface (with function names starting with the `Dk` prefix)
+
+      .. seealso::
+
+         https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/asplos2011-drawbridge.pdf
+
+      Whenever Graphene requires a service from the host platform (memory
+      allocation, thread management and synchronization, file system and network
+      stacks, etc.), it calls the corresponding PAL functionality. The PAL ABI
+      is host-platform agnostic and is backed by the host-platform specific PAL,
+      for example, the Linux-SGX PAL.
 
    SGX
       Software Guard Extensions is a set of instructions on Intel processors.