[development] Custom FAPI elements in 6.x

[Moshe Weitzman]

wow - this is exceptionally clear documentation. worthy of matt and
JVD's book i think. thanks karen.

the only improvement i can think of is more introductory info about
when one might want to use hook_elements(). something like:
"Use hook_elements() to create a new field type or a new property of
forms that all forms can then use. the best example of this hook in
core is system_elements(), which defines all of the core form fields
like textfield, checkbox, etc.)."

i think karoly was referring to the 4 instances of #after_build in core.