[kernel|doc] Updated doc for the new sctp non_block_send option

Add documention for sctp option non_block_send.

OTP-19258

lib/kernel/src/gen_sctp.erl

@@ -133,8 +133,15 @@ by calling `inet:setopts/2`.  They can be retrieved using `inet:getopts/2`.
   larger than `val(recbuf)`. Setting this option also adjusts the size
   of the driver buffer (see `buffer` above).
 
+### [](){: #option_non_block_send }
+
+- **`{non_block_send, boolean()}`** - A send call that would otherwise block (hang),
+  will instead immediately return with e.g. `{error, eagain}`
+  *if* this option has been set to `true`.
+  Defaults to `false`.
+
 - **`{sctp_module, module()}`** - Overrides which callback module is used.
-  Defaults to `inet_sctp` for IPv4 and `inet6_sctp` for IPv6.
+Defaults to `inet_sctp` for IPv4 and `inet6_sctp` for IPv6.
 
 - **`{sctp_rtoinfo, #sctp_rtoinfo{}}`**
 
@@ -1504,6 +1511,14 @@ and context (passed to the local SCTP layer), which can be used,
 for example, for error identification.  However, such a fine grained
 user control is rarely required. The function [`send/4`](`send/4`)
 is sufficient for most applications.
+
+> #### Note {: .info }
+>
+> Send is normally blocking, but if the socket option
+> [`non_block_send`](#option_non_block_send) is set to true,
+> the function will return with e.g. {error, eagain}
+> in the case when the function would otherwise block.
+> It is then up to the user to try again later.
 """.
 -spec send(Socket, SndRcvInfo, Data) -> ok | {error, Reason} when
       Socket :: sctp_socket(),
@@ -1530,6 +1545,14 @@ Sends a `Data` message on the association `Assoc` and `Stream`.
 [`#sctp_assoc_change{}`](#record-sctp_assoc_change) record
 from an association establishment, or as the `t:assoc_id/0`
 `t:integer/0` field value.
+
+> #### Note {: .info }
+>
+> Send is normally blocking, but if the socket option
+> [`non_block_send`](#option_non_block_send) is set to true,
+> the function will return with e.g. {error, eagain}
+> in the case when the function would otherwise block.
+> It is then up to the user to try again later.
 """.
 -spec send(Socket, Assoc, Stream, Data) -> ok | {error, Reason} when
       Socket :: sctp_socket(),