undocumented parts of SPI

I'm trying to figure out the SPI and need a bit of help, because the
docs do not mention (that I can find) some parts of the interface.

- SPI_exec(char * sql, int count): this function seems to execute the
query. Is the `count' parameter the maximum number of tuples to
return? Does count=0 imply return everything? Return value are the
SPI_* flags defined in spi.h?

- SPI_processed: number of tuples processed during execution of
SPI_exec? Does this equate to N in the INSERT/DELETE/UPDATE N
messages that are emitted by psql?

- Are there any restrictions on what types of queries may be executed
by a trigger using SPI_exec?

Thanks for the help.

Cheers,
Brook