Request regarding SquashFS documentation

[Larry Doolittle]

In https://reproducible-builds.org/docs/system-images/ there's a paragraph
about SquashFS metadata & compression.  It's superficially OK, but points
to IMHO an obsolete fork of mksquashfs.  No offense to lynxis!  He did
a good job getting usable tools in our hands early on.  But the main
squashfs-tools project has caught up, specifically version 4.5.1 that's now
in Debian testing.  I've used it (and some predecessor versions) for a while
now, and its output is demonstrably reproducible IRL.  (If I'm wrong, please
let me and the squashfs-tools developers know!)

Here's my attempt to rewrite that paragraph.

When building SquashFS images, older versions of the tools sometimes
yielded unreproducible results.  A good mksquashfs will
 * honor $SOURCE_DATE_EPOCH for various timestamps
 * clamp content timestamps to $SOURCE_DATE_EPOCH
 * not reorder fragments based on multithreading conditions
(squashfs-tools)[https://github.com/plougher/squashfs-tools] 4.5.1 
(in Debian Bookworm) is good here,
having absorbed important features from 
(squashfskit)[https://github.com/squashfskit/squashfskit]

  - Larry