Documentation: nfs: convert pnfs.txt to ReST

Convert pnfs.txt to ReST. Content remains mostly unchanged.

Signed-off-by: Daniel W. S. Almeida <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 386eaad..45d7919 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -51,3 +51,4 @@
    overlayfs
    virtiofs
    vfat
+   nfs/index