Documentation for output/vsplit

And other small and mostly non-impacting code changes.
Bugfix for margin-top=0pt etc. on textformat

When having a text format such as     
    <DefineTextformat name="text" margin-top="0pt" padding-top="0pt"/>
the margin-top and padding-top resulted in a 0-sized glue that allowed a 
frame break. Now we only insert the break iff there the glue is != 0.

This is not a good solution, but works well enough for the trivial
cases.
M: Bugfix for margin-top=0pt etc. on textformat
Re-implement vsplit

A cleaner and saner implementation of vsplit. Improved documentation.