Typo

removed one typo introduced previously and, more importantly, tried to rephrase and slightly restructure the latexmath/mathjax related part of the user guide. mathjax is discussed first and the limitations implied by the mathml route (notably that many browsers simply don' t support it) is mentioned. I think the manual
should leave the (probably correct) impression that the user should leave mathml alone and stay with latex math and mathjax.