docs: filesystems: caching/cachefiles.txt: convert to ReST

- Add a SPDX header;
- Adjust document title;
- Mark literal blocks as such;
- Add table markups;
- Comment out text ToC for html/pdf output;
- Add lists markups;
- Add it to filesystems/caching/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/eec0cfc268e8dca348f760224685100c9c2caba6.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

fs/cachefiles/Kconfig

@@ -8,7 +8,7 @@ config CACHEFILES
 	  filesystems - primarily networking filesystems - thus allowing fast
 	  local disk to enhance the speed of slower devices.
 
-	  See Documentation/filesystems/caching/cachefiles.txt for more
+	  See Documentation/filesystems/caching/cachefiles.rst for more
 	  information.
 
 config CACHEFILES_DEBUG
@@ -36,5 +36,5 @@ config CACHEFILES_HISTOGRAM
 	  bouncing between CPUs.  On the other hand, the histogram may be
 	  useful for debugging purposes.  Saying 'N' here is recommended.
 
-	  See Documentation/filesystems/caching/cachefiles.txt for more
+	  See Documentation/filesystems/caching/cachefiles.rst for more
 	  information.