[DGD] API note for kernel library

[Raymond Jennings]

It is hereby declared that attempting to start more callouts in an
object than the maximum array size during a callout suspension will
have undefined results, and in general it is recommended that an
object not have more callouts than the maximum array size at all.

I observed runtime errors when attempts were made to deal with this in
the middle of a callout suspension.

It's been this way for awhile based on the kernel library's usage of
status() to handle callout suspension, but it's high time this
limitation was publicly documented.