Update GIL documentation

docs/src/userguide/external_C_code.rst

@@ -416,9 +416,10 @@ available by all three methods.
 Acquiring and Releasing the GIL
 ---------------------------------
 
-Cython provides facilities for releasing the Global Interpreter Lock (GIL)
-before calling C code, and for acquiring the GIL in functions that are to be
-called back from C code that is executed without the GIL.
+Cython provides facilities for acquiring and releasing the
+`Global Interpreter Lock (GIL) <http://docs.python.org/dev/glossary.html#term-global-interpreter-lock>`_.
+This may be useful when calling into (external C) code that may block, or when wanting to use Python from a
+C callback.
 
 .. _nogil:
 
@@ -446,10 +447,12 @@ header::
     cdef void my_callback(void *data) with gil:
         ...
 
-If the callback may be called from another thread then the main thread which may not be a Python thread,
-care must be taken to initialize the GIL first, through a call to ``PyEval_InitThreads()``.
+If the callback may be called from another non-Python thread,
+care must be taken to initialize the GIL first, through a call to
+`PyEval_InitThreads() <http://docs.python.org/dev/c-api/init.html#PyEval_InitThreads>`_.
+If you're already using  :ref:`cython.parallel <parallel>` in your module, this will already have been taken care of.
 
-The GIL may also be acquired through the ``with nogil`` counterpart ``with gil``::
+The GIL may also be acquired through the ``with gil`` statement::
 
     with gil:
         <execute this block with the GIL acquired>
@@ -470,6 +473,9 @@ acquiring the GIL first. Some of these restrictions are currently checked by
 Cython, but not all. It is possible that more stringent checking will be
 performed in the future.
 
+.. NOTE:: This declaration declares that it is safe to call the function without the GIL,
+          it does not in itself release the GIL.
+
 Declaring a function with :keyword:`gil` also implicitly makes its signature
 :keyword:`nogil`.
 

docs/src/userguide/parallelism.rst

@@ -2,6 +2,8 @@
 
 .. py:module:: cython.parallel
 
+.. _parallel:
+
 **********************************
 Using Parallelism
 **********************************
@@ -11,7 +13,8 @@ module. To use this kind of parallelism, the GIL must be released
 (see :ref:`Releasing the GIL <nogil>`).
 It currently supports OpenMP, but later on more backends might be supported.
 
-.. NOTE:: Because the backend is OpenMP, cython.parallel functionality may only be used from the main thread or from OpenMP threads.
+.. NOTE:: Functionality in this module may only be used from the main thread
+          or parallel regions due to OpenMP restrictions.
 
 __ nogil_