api: Document A18 TCP_USER_TIMEOUT handling for keepalive

[ejona86]

Fixes #12487

CC @Trajanv

[Trajanv]

gRPC keepalive documentation: gRPC Keepalive guide. The table describes:

KEEPALIVE_TIMEOUT — Client and Server — The timeout in milliseconds for a PING frame to be acknowledged. If sender does not receive an acknowledgment within this time, it will close the connection.

Given that gRPC-Java may trigger a connection closure via TCP_USER_TIMEOUT even without sending a PING frame, i think, this description needs an update to reflect that behavior? Specifically, that any unacknowledged outbound packet (DATA or PING) can lead to connection termination when TCP_USER_TIMEOUT is in effect, independent of the PING-based timeout.

We just want to make sure users understand the potential for RSTs on long-lived streams when keepAliveTimeout is very low.