comark/annotate/missing-alt · Describe every ::screenshot in its alt

alt is what a screen reader says for the screenshot, and what shows when the image doesn't load. Without it, a reader who can't see the image doesn't know what the marks are on. Write what the screenshot shows.

Incorrect

::screenshot{src="/pond.png"}
::

lint-comark example.mdexample.md:1:19 comark/annotate/missing-altscreenshot "/pond.png": alt is required.Describe what the screenshot shows.✖ 1 problem

Correct

::screenshot{src="/pond.png" alt="The pond map with three ducks"}
::

lint-comark example.md✔ No problems