docs/mm: Physical Memory: remove useless markup

Jon says:

> +See also :ref:`Page Reclaim <page_reclaim>`.

Can also just be "See also Documentation/mm/page_reclaim.rst". The
right things will happen in the HTML output, readers of the plain-text
will know immediately where to go, and we don't have to add the label
clutter.

Remove reference markup and unnecessary labes and use plain file names.

Fixes: 5d8c5e430a63 ("docs/mm: Physical Memory: add structure, introduction and nodes description")
Suggested-by: Jonathan Corbet <[e-mail]>
Acked-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Mike Rapoport (IBM) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/dma-api.rst

···
-.. _dma_api:
-
 ============================================
 Dynamic DMA mapping using the generic device
 ============================================

Documentation/mm/page_reclaim.rst

···
 .. SPDX-License-Identifier: GPL-2.0
 
-.. _page_reclaim:
-
 ============
 Page Reclaim
 ============

Documentation/mm/physical_memory.rst

···
 * ``ZONE_DMA`` and ``ZONE_DMA32`` historically represented memory suitable for
   DMA by peripheral devices that cannot access all of the addressable
   memory. For many years there are better more and robust interfaces to get
-  memory with DMA specific requirements (:ref:`DMA API <dma_api>`), but
-  ``ZONE_DMA`` and ``ZONE_DMA32`` still represent memory ranges that have
+  memory with DMA specific requirements (Documentation/core-api/dma-api.rst),
+  but ``ZONE_DMA`` and ``ZONE_DMA32`` still represent memory ranges that have
   restrictions on how they can be accessed.
   Depending on the architecture, either of these zone types or even they both
   can be disabled at build time using ``CONFIG_ZONE_DMA`` and
···
   change, their content may move between different physical pages. Often
   ``ZONE_MOVABLE`` is populated during memory hotplug, but it may be
   also populated on boot using one of ``kernelcore``, ``movablecore`` and
-  ``movable_node`` kernel command line parameters. See :ref:`Page migration
-  <page_migration>` and :ref:`Memory Hot(Un)Plug <admin_guide_memory_hotplug>`
-  for additional details.
+  ``movable_node`` kernel command line parameters. See
+  Documentation/mm/page_migration.rst and
+  Documentation/admin-guide/mm/memory_hotplug.rst for additional details.
 
 * ``ZONE_DEVICE`` represents memory residing on devices such as PMEM and GPU.
   It has different characteristics than RAM zone types and it exists to provide
···
 Reclaim control
 ~~~~~~~~~~~~~~~
 
-See also :ref:`Page Reclaim <page_reclaim>`.
+See also Documentation/mm/page_reclaim.rst.
 
 ``kswapd``
   Per-node instance of kswapd kernel thread.