xz: Update the man page about --block-size.
diff --git a/src/xz/xz.1 b/src/xz/xz.1
index 49b0778..a006bc4 100644
--- a/src/xz/xz.1
+++ b/src/xz/xz.1
@@ -5,7 +5,7 @@
.\" This file has been put into the public domain.
.\" You can do whatever you want with this file.
.\"
-.TH XZ 1 "2013-11-12" "Tukaani" "XZ Utils"
+.TH XZ 1 "2014-12-16" "Tukaani" "XZ Utils"
.
.SH NAME
xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
@@ -826,14 +826,39 @@
format, split the input data into blocks of
.I size
bytes.
-The blocks are compressed independently from each other.
-.\" FIXME: Explain how to these can be used for random access and threading.
+The blocks are compressed independently from each other,
+which helps with multi-threading and
+makes limited random-access decompression possible.
+This option is typically used to override the default
+block size in multi-threaded mode,
+but this option can be used in single-threaded mode too.
.IP ""
-In multi-threaded mode the sizes of the blocks
-are stored in the block headers.
-This isn't done in single-threaded mode,
-so the encoded output won't be
-identical to that of the multi-threaded mode.
+In multi-threaded mode about three times
+.I size
+bytes will be allocated in each thread for buffering input and output.
+The default
+.I size
+is three times the LZMA2 dictionary size or 1 MiB,
+whichever is more.
+Typically a good value is 2\-4 times
+the size of the LZMA2 dictionary or at least 1 MiB.
+Using
+.I size
+less than the LZMA2 dictionary size is waste of RAM
+because then the LZMA2 dictionary buffer will never get fully used.
+The sizes of the blocks are stored in the block headers,
+which a future version of
+.B xz
+will use for multi-threaded decompression.
+.IP ""
+In single-threaded mode no block splitting is done by default.
+Setting this option doesn't affect memory usage.
+No size information is stored in block headers,
+thus files created in single-threaded mode
+won't be identical to files created in multi-threaded mode.
+The lack of size information also means that a future version of
+.B xz
+won't be able decompress the files in multi-threaded mode.
.TP
.BI \-\-block\-list= sizes
When compressing to the
