docs: kernel-hacking: hacking.rst: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9537b74d897fab13552535d79337060a3b241b8c.1623824363.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-hacking/hacking.rst

@@ -601,7 +601,7 @@ Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-:doc:`../core-api/symbol-namespaces`
+Documentation/core-api/symbol-namespaces.rst
 
 :c:func:`EXPORT_SYMBOL_NS_GPL()`
 --------------------------------
@@ -610,7 +610,7 @@ Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
-:doc:`../core-api/symbol-namespaces`
+Documentation/core-api/symbol-namespaces.rst
 
 Routines and Conventions
 ========================