Docs: improve use of JSDoc

The sharp source code and therefore its API is already relatively well documented using JSDoc, but we can always improve.

Commit https://github.com/lovell/sharp/commit/ef849fd63907d937afce0d06b07ae778692ec500 switched the logic to generate markdown documentation for the API from documentationjs to the better-maintained jsdoc2md. This took a very basic approach to migration and didn't touch any source files, however there are future possible improvements we can now take advantage of.

• Generate a single abstract syntax tree (AST) from all source files, then write multiple output files by filtering this AST by source file.
• Add @module, @class, @memberOf etc. annotions for correct inheritance and return types, to help prevent the "global" label.
• Add @chainable annotations where relevant, which is an extension of JSDoc supported by jsdoc-parse.
• Caption all code examples via @example <caption>Explain the example here</caption>.

Begin with commit ef849fd63907d937afce0d06b07ae778692ec500 and inspect the JSDoc in the sharp source files alongside the current jsdoc2md generation. Choose a specific improvement from the listed possibilities, then verify that the generated API markdown reflects it and existing documentation generation still succeeds.