Deprecate @synopsis tag

Since we moved the parameter descriptions inline (#548), I noticed that the information is split between the `@synopsis` tag and the longdesc section:

* the synopsis contains optionality (optional/mandatory) and cardinality (one, many)
* the longdesc contains the description
* both contain the key (and value) name

The duplication is annoying and the `@synopsis` tag tends to get too long.

Would it be feasible to move (and then parse back out) all the info to the longdesc section and deprecate the `@synopsis` tag?