Double quotes (“example”) are entered like this:

<quote>example</quote>

Exception: In program listings, they're written verbatim:

"example"

The difference between “example” and "example" might not be visible in every output format (particularly in HTML). For printed material, however, it's important to use the <quote> element wherever appropriate.

For writing curly-quoted single literal characters, you should use the ‘ and ’XML entities. Example:

Terminate an SQL statement with a &lsquo;<literal>;</literal>&rsquo; character.

That example will look like this:

Terminate an SQL statement with a “;” character.

Curly single quotes is a pretty common convention with O'Reilly and Pearson.

Single quotes (') and backticks (`) are always entered verbatim.

http://docbook.org/tdg/en/html/quote.html