xqdoc: add schema import info

In the documentation of the modules that import schemas please give an easy way for people
to do copy and paste the schema import statements (the same way we do with the module
import).

E.g.

add a text like:

"Please note that the schemas are not automatically imported in the modules that import this
module. In order to import and use the schemas, please add:

import schema "http://www.zorba-xquery.com/modules/full-text"; (OR WHATEVER SCHEMA)

under the section

Imported schemas:
http://www.zorba-xquery.com/modules/full-text