API Documentation fixes mainly in libmmgs.h, and a workaround for a Doxygen problem

[mpotse]

• Added a \brief for every function in libmmgs.h
• The usual language fixes
• used \ref to get links for types and functions that appear in documentation
• wrote a bit more detail about some functions
• workaround for a Doxygen problem in common/inout.c : it saw an if() with a long function call in the condition as a function call itself and added "if" to the call graphs as if it was a function. I think it's also easier to read now. This should be my last grocery-list kind of PR, I'll get to more concentrated changes.