docs: x86: 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`.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/17c68b5f1d72488431c77c1de9f13683fe9f536c.1623824363.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/boot.rst

@@ -1343,7 +1343,7 @@ follow::
 In addition to read/modify/write the setup header of the struct
 boot_params as that of 16-bit boot protocol, the boot loader should
 also fill the additional fields of the struct boot_params as
-described in chapter :doc:`zero-page`.
+described in chapter Documentation/x86/zero-page.rst.
 
 After setting up the struct boot_params, the boot loader can load the
 32/64-bit kernel in the same way as that of 16-bit boot protocol.
@@ -1379,7 +1379,7 @@ can be calculated as follows::
 In addition to read/modify/write the setup header of the struct
 boot_params as that of 16-bit boot protocol, the boot loader should
 also fill the additional fields of the struct boot_params as described
-in chapter :doc:`zero-page`.
+in chapter Documentation/x86/zero-page.rst.
 
 After setting up the struct boot_params, the boot loader can load
 64-bit kernel in the same way as that of 16-bit boot protocol, but

Documentation/x86/mtrr.rst

@@ -28,7 +28,7 @@ are aligned with platform MTRR setup. If MTRRs are only set up by the platform
 firmware code though and the OS does not make any specific MTRR mapping
 requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.
 
-For details refer to :doc:`pat`.
+For details refer to Documentation/x86/pat.rst.
 
 .. tip::
   On Intel P6 family processors (Pentium Pro, Pentium II and later)