xz: Update the man page about --threads.

src/xz/xz.1

diff --git a/src/xz/xz.1 b/src/xz/xz.1
index a006bc4..75aead3 100644
--- a/src/xz/xz.1
+++ b/src/xz/xz.1
@@ -1055,11 +1055,6 @@
 can be overriden with the
 .BI \-\-block\-size= size
 option.
-.IP ""
-.B "It is possible that the details of this option change before"
-.B "the next stable XZ Utils release."
-.B "This may include the meaning of the special value 0."
-.\" FIXME
 .
 .SS "Custom compressor filter chains"
 A custom filter chain allows specifying