Adds synopses for those commands missing them.
Moves all synopsis sections to code blocks. This improves the appearance, although highlighting as
fish code may not be the ideal appearance.
Someone has hit the 10MiB limit (and of course it's the number of
javascript packages), and we don't handle it fantastically currently.
And even though you can't pass a variable of that size in one go, it's
plausible that someone might do it in multiple passes.
See #5267.
We were flip-flopping between the two terms, so we now use one. We
still mention "array" in the chapter, and it's still `read --array`,
though.
Fixes#5846.
[ci skip]
