xz: Document the "summary" lines of --robot -lvv.

This documents only the columns that are in v5.0.
The new columns added in the master branch aren't
necessarily stable yet.

src/xz/xz.1

@@ -1836,6 +1836,25 @@ headers.
 .PD
 .PP
 The columns of the
+.B summary
+lines:
+.PD 0
+.RS
+.IP 2. 4
+Amount of memory (in bytes) required to decompress
+this file with this
+.B xz
+version
+.IP 3. 4
+.B yes
+or
+.B no
+indicating if all block headers have both compressed size and
+uncompressed size stored in them
+.RE
+.PD
+.PP
+The columns of the
 .B totals
 line:
 .PD 0