docs: Unbreak the PS/PDF builds for the User's Guide (Trac #10509)
dblatex can only translate the Unicode glyphs introduced in Trac #10509 for LaTeX
if the latex.unicode.use=1 flag is set, otherwise it will just fail.
However, note that adding this flag is not going to fully solve the problem as
those symbols are not known by LaTeX, so the corresponding character codes
will be added instead to the resulting PS/PDF files. Hence it is considered
an interim solution only, not a true fix, until a better one is found.