Incorrect image description for Session mode example

[Andre053]

Source: https://docs.syncthing.net/specs/relay-v1.html#id1

The Example Exchange caption references permanent and temporary protocol mode instead of session mode. Additionally, I think a deliminator between Client A and Client B descriptions would increase clarity. I propose a change to:

Client A - Session mode sender; Client B - Session mode receiver

Must add the deliminator to other descriptions for consistency.