doc: update after the sync request change

update documentation after commit c9325a22.
("fix bidirectional event confusion")

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
doc: improve RyuApp documentation

This includes a small code change; the default value of OFP_VERIONS
was changed to None from the lack of the attribute.  No functional
changes are intended by this.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>