xz: Document behavior of --block-list with threads.

This needs to be updated before 5.2.0.

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 "2013-10-22" "Tukaani" "XZ Utils"
+.TH XZ 1 "2013-10-25" "Tukaani" "XZ Utils"
 .
 .SH NAME
 xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
@@ -832,8 +832,26 @@ A special value of
 may be used as the last value to indicate that
 the rest of the file should be encoded as a single block.
 .IP ""
-.B "Currently this option is badly broken if used together with"
-.B "\-\-block\-size or with multithreading."
+If this option is used in threaded mode and one specifies
+.I sizes
+that exceed the encoder's block size
+(either the default value or the value specified
+with \fB\-\-block\-size=\fIsize\fR),
+the encoder will create additional blocks while
+keeping the boundaries specified in
+.IR sizes .
+For example, if one specifies
+.B \-\-threads=2
+.B \-\-block\-size=10MiB
+.B \-\-block\-list=5MiB,10MiB,8MiB,12MiB,24MiB
+and the input file is 80 MiB,
+one will get 11 blocks:
+5, 10, 8, 10, 2, 10, 10, 4, 10, 10, and 1 MiB.
+.IP ""
+.\" FIXME
+.B "In single-threaded mode \-\-block\-size is ignored"
+.B "if \-\-block\-list is also specified."
+.B "This might change before 5.2.0 is released."
 .TP
 .BI \-\-flush\-timeout= timeout
 When compressing, if more than