[BUG] manual should explain how to insert literal dollar sign

[Paul Rubin]

Context: I wanted to mention in a document that a certain item cost 
"$12.98+$.77 s/h".  Since the period after the second $ is punctuation, 
the parsing heuristic interpreted that as LaTeX, giving math output in 
the html export.  So I spent a while muddling around trying to figure 
out how to escape the dollar sign, then sought help on IRC, where I got 
the answer: write \dollar and the Org entity parser will turn it into a 
literal dollar sign.  I would not have figured this out myself.

Requested fix: explain the \dollar escape in the Org manual section 
"LaTeX fragments".

Thanks!

Paul