Suggested by Maxime Devos <maximedevos@telenet.be>
and Maxim Cournoyer <maxim.cournoyer@gmail.com>.
* doc/guix.texi (package Reference): Clarify 'native-search-paths'
vs. 'search-paths'.
(Search Paths): Link to it. Remove unnecessarily "define libxml2".
Reword 'file-pattern' description that said "When true".