General Documentation Improvements

The documentation of jsPDF is not very good. Many methods, classes, and parameters are badly documented or not at all.

• Move all JSDoc comments to the typings file. This will improve the quality of the typings significantly, because IDEs can then display documentation inline. Change the jsdoc task so it uses the typings file instead of the source files (package.json script)
• Review, fix, and update all documentation comments. Verify the behavior with the code. If there are questions, ask me ;)