INSTALL,php/docs/index.rst: In current PHP releases, dl() will only
look in the configured extension directory, and only accepts a
leafname, and dl() is disabled in most SAPIs, so update the
documentation about how to install the PHP bindings without being
root.  Fixes #661, reported by Felix Ostmann.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18320 6b2f1b92-a4f3-0310-943f-de3cbe41748b