xz.txt: standardize document format

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:
- Use marks for titles;
- Adjust indentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/xz.txt

@@ -1,8 +1,9 @@
-
+============================
 XZ data compression in Linux
 ============================
 
 Introduction
+============
 
 XZ is a general purpose data compression format with high compression
 ratio and relatively fast decompression. The primary compression
@@ -22,6 +23,7 @@ Introduction
 <http://tukaani.org/xz/>.
 
 XZ related components in the kernel
+===================================
 
 The xz_dec module provides XZ decompressor with single-call (buffer
 to buffer) and multi-call (stateful) APIs. The usage of the xz_dec
@@ -52,6 +54,7 @@ XZ related components in the kernel
 which will use no BCJ filter and 1 MiB LZMA2 dictionary.
 
 Notes on compression options
+============================
 
 Since the XZ Embedded supports only streams with no integrity check or
 CRC32, make sure that you don't use some other integrity check type
@@ -73,7 +76,7 @@ Notes on compression options
 by the kernel. 1 MiB is probably the maximum reasonable dictionary
 size for in-kernel use (maybe more is OK for initramfs). The presets
 in XZ Utils may not be optimal when creating files for the kernel,
-    so don't hesitate to use custom settings. Example:
+so don't hesitate to use custom settings. Example::
 
 	xz --check=crc32 --lzma2=dict=512KiB inputfile
 
@@ -85,6 +88,7 @@ Notes on compression options
 be at least as big as the uncompressed data itself.
 
 Future plans
+============
 
 Creating a limited XZ encoder may be considered if people think it is
 useful. LZMA2 is slower to compress than e.g. Deflate or LZO even at
@@ -97,6 +101,7 @@ Future plans
 outside the Linux kernel.
 
 Conformance to the .xz file format specification
+================================================
 
 There are a couple of corner cases where things have been simplified
 at expense of detecting errors as early as possible. These should not
@@ -105,6 +110,7 @@ Conformance to the .xz file format specification
 from XZ Utils.
 
 Reporting bugs
+==============
 
 Before reporting a bug, please check that it's not fixed already
 at upstream. See <http://tukaani.org/xz/embedded.html> to get the