docs: move nommu-mmap.txt to admin-guide and rename to ReST

The nommu-mmap.txt file provides description of user visible
behaviuour. So, move it to the admin-guide.

As it is already at the ReST, also rename it.

Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3a63d1833b513700755c85bf3bda0a6c4ab56986.1592918949.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

init/Kconfig

@@ -1957,7 +1957,7 @@ config MMAP_ALLOW_UNINITIALIZED
 	  userspace.  Since that isn't generally a problem on no-MMU systems,
 	  it is normally safe to say Y here.
 
-	  See Documentation/nommu-mmap.txt for more information.
+	  See Documentation/mm/nommu-mmap.rst for more information.
 
 config SYSTEM_DATA_VERIFICATION
 	def_bool n