Notes on what *SUBSCRIBE returns (or doesn't return) (#2327)

Add return info to *subscribe and *unsubscribe commands.

Add some notes about RESP3.

commands/psubscribe.md

@@ -7,3 +7,17 @@ Supported glob-style patterns:
 * `h[ae]llo` subscribes to `hello` and `hallo,` but not `hillo`
 
 Use `\` to escape special characters if you want to match them verbatim.
+
+Once the client enters the subscribed state it is not supposed to issue any other commands, except for additional `SUBSCRIBE`, `SSUBSCRIBE`, `PSUBSCRIBE`, `UNSUBSCRIBE`, `SUNSUBSCRIBE`, `PUNSUBSCRIBE`, `PING`, `RESET` and `QUIT` commands.
+However, if RESP3 is used (see `HELLO`) it is possible for a client to issue any commands while in subscribed state.
+
+For more information, see [Pub/sub](/docs/manual/pubsub/).
+
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each pattern, one message with the first element being the string "psubscribe" is pushed as a confirmation that the command succeeded.
+
+## Behavior change history
+
+*   `>= 6.2.0`: `RESET` can be called to exit subscribed state.

commands/punsubscribe.md

@@ -5,3 +5,8 @@ When no patterns are specified, the client is unsubscribed from all the
 previously subscribed patterns.
 In this case, a message for every unsubscribed pattern will be sent to the
 client.
+
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each pattern, one message with the first element being the string "punsubscribe" is pushed as a confirmation that the command succeeded.

commands/ssubscribe.md

@@ -7,6 +7,12 @@ A client can subscribe to channels across different slots over separate `SSUBSCR
 
 For more information about sharded Pub/Sub, see [Sharded Pub/Sub](/topics/pubsub#sharded-pubsub).
 
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each shard channel, one message with the first element being the string "ssubscribe" is pushed as a confirmation that the command succeeded.
+Note that this command can also return a -MOVED redirect.
+
 @examples
 
 ```

commands/subscribe.md

@@ -3,7 +3,15 @@ Subscribes the client to the specified channels.
 Once the client enters the subscribed state it is not supposed to issue any
 other commands, except for additional `SUBSCRIBE`, `SSUBSCRIBE`, `PSUBSCRIBE`, `UNSUBSCRIBE`, `SUNSUBSCRIBE`, 
 `PUNSUBSCRIBE`, `PING`, `RESET` and `QUIT` commands.
+However, if RESP3 is used (see `HELLO`) it is possible for a client to issue any commands while in subscribed state.
+
+For more information, see [Pub/sub](/docs/manual/pubsub/).
+
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each channel, one message with the first element being the string "subscribe" is pushed as a confirmation that the command succeeded.
 
 ## Behavior change history
 
-*   `>= 6.2.0`: `RESET` can be called to exit subscribed state.
+*   `>= 6.2.0`: `RESET` can be called to exit subscribed state.

commands/sunsubscribe.md

@@ -6,3 +6,8 @@ In this case a message for every unsubscribed shard channel will be sent to the
 Note: The global channels and shard channels needs to be unsubscribed from separately.
 
 For more information about sharded Pub/Sub, see [Sharded Pub/Sub](/topics/pubsub#sharded-pubsub).
+
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each shard channel, one message with the first element being the string "sunsubscribe" is pushed as a confirmation that the command succeeded.

commands/unsubscribe.md

@@ -5,3 +5,8 @@ When no channels are specified, the client is unsubscribed from all the
 previously subscribed channels.
 In this case, a message for every unsubscribed channel will be sent to the
 client.
+
+@return
+
+When successful, this command doesn't return anything.
+Instead, for each channel, one message with the first element being the string "unsubscribe" is pushed as a confirmation that the command succeeded.