docs: filesystems: convert ubifs-authentication.rst.txt to ReST

- Add a SPDX header;
- Mark some literals as such;
- Add it to filesystems/index.rst.
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0c36091b6660cd372f994bd98e1264491d766c22.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -90,5 +90,6 @@ Documentation for filesystem implementations.
    sysfs
    sysv-fs
    tmpfs
+   ubifs-authentication.rst
    virtiofs
    vfat

Documentation/filesystems/ubifs-authentication.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 :orphan:
 
 .. UBIFS Authentication
@@ -92,11 +94,11 @@ UBIFS Index & Tree Node Cache
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Basic on-flash UBIFS entities are called *nodes*. UBIFS knows different types
-of nodes. Eg. data nodes (`struct ubifs_data_node`) which store chunks of file
-contents or inode nodes (`struct ubifs_ino_node`) which represent VFS inodes.
-Almost all types of nodes share a common header (`ubifs_ch`) containing basic
+of nodes. Eg. data nodes (``struct ubifs_data_node``) which store chunks of file
+contents or inode nodes (``struct ubifs_ino_node``) which represent VFS inodes.
+Almost all types of nodes share a common header (``ubifs_ch``) containing basic
 information like node type, node length, a sequence number, etc. (see
-`fs/ubifs/ubifs-media.h`in kernel source). Exceptions are entries of the LPT
+``fs/ubifs/ubifs-media.h`` in kernel source). Exceptions are entries of the LPT
 and some less important node types like padding nodes which are used to pad
 unusable content at the end of LEBs.