I just checked. The only presence of this feature in the Bash man page is under REDIRECTION with the following, as the fifth and sixth elements of the list:

     /dev/tcp/host/port
       If host is a valid hostname or Internet address, and port is an integer
       port number or service name, bash attempts to open the corresponding
       TCP socket.
     /dev/udp/host/port
       If host is a valid hostname or Internet address, and port is an integer
       port number or service name, bash attempts to open the corresponding
       UDP socket.

It's not a feature with a large documentation footprint. I've known about it for a while, but mostly only in the context of security too. I have wondered whether the majority use of this feature is to provide hacked network shells before. It's probably a feature that never should have been added, though I understand the initial appeal.