mathml: document lsm_mathml_element_update.
mathml: document lsm_mathml_element_update.

gjtorikian/lasem,gjtorikian/lasem,wavewave/lasem,gjtorikian/lasem,janmarthedal/mathml2svg,gjtorikian/lasem,janmarthedal/mathml2svg,janmarthedal/mathml2svg,wavewave/lasem,janmarthedal/mathml2svg,wavewave/lasem,wavewave/lasem