Remove some non-ascii characters.

Remove some non-ascii characters.

My copy of pdflatex does not like (i.e., I cannot compile the doxygen-generated
file _formulas.tex) math statements with unicode MINUS SIGN instead of the old
ASCII HYPHEN-MINUS. While here I also switched out RIGHT SINGLE QUOTATION MARK
for (also ASCII) APOSTROPHE and made a few other small changes.

We should be, in general, able to use unicode in our documentation outside of
math mode so I didn't change those usages unless they seemed gramatically
incorrect to me (like using the right quotation mark instead of an apostrophe).