Channels clarifiaction

[PiotrKorkus]

Add documentation for channel spsc, especially:

• Explain behavior when queue is full.
• Explain when the channel is closed.

Add documentation for channel spmc_broadcast, especially:

• Explain behavior when queue is full. Highlight the case where only one consumer bottleneck reads.
• Specify that Err(CommonErrors::GenericError) will be returned for the send when last consumer is dropped.
• Explain when the channel is closed.
• Clarify send documentation part:
  • Err(CommonErrors::GenericError) - When all Receivers are disconnected (i.e., dropped) was accounted at least on single receiver.
  • all receivers or at least one?