Update info about superscripts and subscripts

@mhoeber @lamagnifica @catong Please review. If it looks good, go ahead and merge it - I'll be in transit most of the day on Monday so I won't be able to check in until late afternoon/early evening.
if there is time, I'd prefer to see these bullets reformatted to put the subject first. For example,
* **Exponents or superscripts**: Insert a caret (^) before an exponent or superscript. If the exponent or superscript includes multiple characters or is an expression, surround the expression with parentheses. For example, enter ``x^n`` or ``x^(n-1)``.
* **Subscripts**: Insert an underscore (_) before a subscript...