Fix the description of finfo_open()'s behaviour when no magic file or MAGIC
environment variable are provided.

Additionally, improved the wording of the warning and added a note recommending
the use of the bundled magic database, per Matthew's suggestion in the bug
tracker.

Fixes doc bug #64793 (finfo_open raises warning with env magic file).


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@330245 c90b9560-bf6c-de11-be94-00142212c4b1