It looks like you're using the Markua spec, not the Markua manual.
In the Markua 0.29 spec, alt-caption has been renamed to alt-title.
However, neither alt-caption nor alt-title have ever been supported in Leanpub. We will support alt-title in our Markua 0.29 beta when we ship it.
(We currently treat the stuff in the square brackets as the caption all the time, if present.)
For the current version of Markua supported by Leanpub, please see: