Clarify the intention that interfaces and upper layers use
regions rather than managers directly.

Rearrange API documentation to better group the API functions
used to create FPGA mgr/bridge/regions and the API used for
programming FPGAs.

Signed-off-by: Alan Tull
Suggested-by: Federico Vaga
Acked-by: Moritz Fischer
Signed-off-by: Greg Kroah-Hartman