Test-runner: update silverbox protocol docs.
Test-runner: update silverbox protocol docs.

Update silverbox protocol description to use BNF.
Add more comments.
Explain semantics of protocol fields and restrictions
on field content.

mejedi/tarantool,rtsisyk/tarantool,vasilenkomike/tarantool,nvoron23/tarantool,ocelot-inc/tarantool,dkorolev/tarantool,guard163/tarantool,condor-the-bird/tarantool,ocelot-inc/tarantool,Sannis/tarantool,KlonD90/tarantool,Sannis/tarantool,vasilenkomike/tarantool,KlonD90/tarantool,condor-the-bird/tarantool,rtsisyk/tarantool,dkorolev/tarantool,condor-the-bird/tarantool,vasilenkomike/tarantool,mejedi/tarantool,Sannis/tarantool,KlonD90/tarantool,guard163/tarantool,condor-the-bird/tarantool,dkorolev/tarantool,mejedi/tarantool,nvoron23/tarantool,nvoron23/tarantool,guard163/tarantool,nvoron23/tarantool,dkorolev/tarantool,condor-the-bird/tarantool,vasilenkomike/tarantool,KlonD90/tarantool,vasilenkomike/tarantool,guard163/tarantool,Sannis/tarantool,nvoron23/tarantool,dkorolev/tarantool,rtsisyk/tarantool,KlonD90/tarantool,ocelot-inc/tarantool,mejedi/tarantool,Sannis/tarantool,nvoron23/tarantool,rtsisyk/tarantool,ocelot-inc/tarantool,guard163/tarantool