builtin.h: take over documentation from api-builtin.txt

builtin.h: take over documentation from api-builtin.txt

Delete Documentation/technical/api-builtin.txt and move its content
into builtin.h. Format it as a comment. Remove a '+' which was needed
when the information was formatted for AsciiDoc. Similarly, change
"::" to ":".

Document SUPPORT_SUPER_PREFIX, thereby bringing the documentation up to
date with the available flags.

While at it, correct '3 more things to do' to '4 more things to do'.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>