:doc `(:=)` was not very helpful. This has now been expanded, and the
expanded version lifted out to its own string so as to be (re)used in
both `:=` and `let`'s documentation.

With thanks to skykanin, buzden, gallais, and MithicSpirit on Discord
for inspiration to refactor these docs and explaining the bits I weren't
clear on.

The type annotation matters for those two, it turns out.

Based on feedback from buzden and emdash. Thank you both for looking
things through!  : )

Co-authored-by: G. Allais <guillaume.allais@ens-lyon.org>