docs: pdfdocs: Teach xeCJK about character classes of quotation marks

Quotation marks in "KR" and "JP" variants of Noto CJK fonts are half
width.
xeCJK assumes they are full width by default and does excessive kerning
around them in Korean and Japanese translations.

Give xeCJK proper hints by the \xeCJKDeclareCharClass command.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/0da9e3c5-2716-f576-1df5-2f28ea69f0e8@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>