The system extension "form" comes with a manual in
typo3/sysext/form/Documentation/Manual/en/manual.sxw. As the corresponding
'docPath' key is not defined in ext_emconf.php, the link to open this
manual in not shown in the Extension Manager.

Change-Id: Ia644d3dcebdc8e8909a0a54a760825028e637104
Fixes: #30444
Releases: 4.6
Reviewed-on: http://review.typo3.org/5476
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert