Improve FormLayout API documentation
This fixes a few typos, attempts to provide better links and "see also", and clarifies the use of some properties, especially Kirigami.FormData.buddyFor which was very confusing and required me taking a look at lxr.
I also noticed that the current docs point to a KStars class instead of QtQuick.Item when referring to Item.
This needs testing before merge, I never used Doxygen and KApiDox and simply made these changes by hand in Kate.