docs: filesystems: caching/netfs-api.txt: convert it to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/caching/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cfe4cb1bf8e1f0093d44c30801ec42e74721e543.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/caching/index.rst b/Documentation/filesystems/caching/index.rst
index f488747..d0651db 100644
--- a/Documentation/filesystems/caching/index.rst
+++ b/Documentation/filesystems/caching/index.rst
@@ -8,3 +8,4 @@
 
    fscache
    object
+   netfs-api