Document history, especially the relation to Lwt from unison

[gdt]

From looking at the manual, the history of Lwt is unclear. Assuming it's true, docs should explain (super briefly) how Lwt came to be a separate library from Lwt in unison. They should explain significant changes, allowing the reader to reason about the conflicting views:

• unison bundles an old version of Lwt
• this repo is a fork of the Lwt that originated in unison, and has made incompatible changes. While that's fine, it's therefore not true that unison is bundling an old version.

For context see

• document that unison's lwt is not an old bundled version of the newer thing also called lwt bcpierce00/unison#1032
• https://bugzilla.redhat.com/show_bug.cgi?id=2277254#c2