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.
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]
As a simple replacement for `wc -l`.
This counts both lines on stdin _and_ arguments.
So if "file" has three lines, then `count a b c < file` will print 6.
And since it counts newlines, like wc, `echo -n foo | count` prints 0.
