Suggest documentation for syntax highlighting issues on Windows

[yakatz]

Suggest documentation to preemptively address user issues like #6704

[jgm]

Instead of this, I'd suggest changing

    pandoc --print-highlight-style pygments > my.theme

to

    pandoc -o my.theme --print-highlight-style pygments

(Indeed, if there is anywhere else in the manual where we recommend shell redirection (>), we might want to consider a similar change.)

[yakatz]

Tested and made suggested change

[jgm]

thanks!