Warn about shell-expansion in the docstring of org-latex-to-html-convert-command

[Martin Edström]

I've just been struggling with my custom setting for
`org-latex-to-html-convert-command` outputting many math snippets
wrong. The fault was mine: I didn't correctly shell-quote the input.
I propose to add a warning in the docstring, because many people will
trip the same problem.

The thing is that double-quotes don't work in shell commands.  I had
\"%i\", but it should've been '%i':

(setopt org-latex-to-html-convert-command "node
/home/kept/pub/texToMathML.js \"%i\"")

Math snippets that start with $, like $y = 200$ of course get
butchered into just " = 200$" because of course the first $y gets
interpreted as a shell environment variable.