Modify the type of HolKernel.syntax_fns and add some documentation.

This is in response to issue #247.

The change to the type makes things a little more natural when using partial applications - for example, HolKernel.syntax_fns1 can be used to provide syntax support for regular unary operations.

Improvements to updateLib. Added some comments describing the various conversions.

Note that the interface has changed a bit. Some conversions now take a compset (and/or a conversion) instead of a list of theorems.