Backport change from trunk:
docs/overview.rst: Note that pdftotext is part of poppler as well as
xpdf.  (Noted by Paul Wise)

git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18297 6b2f1b92-a4f3-0310-943f-de3cbe41748b