genlib.fifo: fix docstring section syntax
fhdl.structure: document the API
doc/conf.py: no modindex, no numpydoc warnings

* do not generate boring modindex for html and latex outputs
* numpydoc_show_class_members=False, to skip undocumented methods
  https://github.com/phn/pytpm/issues/3#issuecomment-12133978
fhdl/structure: clarify usage restrictions of LHS Cat
doc/api, index: add fhdl.structure, add index/search

genindex is needed to generate links between members and search is
useful for the html docs.