fix nginx documentation to one that actually works :-)

example configuration wuldn't allow to install silverstripe, as
install.php does exist as a regular file (and that was ignored in the
old version of the documentation)
Similarily, the last rule in the htaccess snippet that should allow the
access to the tinymce php files were never applied, as a previously
listed regex did match and denied access. Even if it would have taken
effect: as those files do exist on disk, they would have been handed out
as-is and not been interpreted by php.
Also the statement regarding accidental/exploitable execution of
arbitrary php was misleading (and to some degree even wrong) in the old
context.

squashed commit as per pr#1791