Update WiFiServer docs for ::write(all clients)

[earlephilhower]

Fixes #5116
Fixes #2743

The Arduino WiFiClient object allows sending the same packet to all
connected clients of a WiFiServer. In their implementation it may make
sense, but on the 8266 with things like SSL it doesn't.

Update the docs to note that WiFiServer::write() is a no-op, and that
the app should use the WiFiCliebnt::write() on all connected clients as
appropriate.

[d-a-v]

This call is impossible to cleanly implement in any environment without a heavy infrastructure (even glibc doesn't have it).

addition:
Why is multicast existing with UDP and not TCP ?
That would be the answer to wifiserver.write() implementation complexity

[JAndrassy]

why implement Arduino Server class at all? there is no use case where a server object is useful as Server.

[d-a-v]

@JAndrassy
WiFiServer is needed to create new WiFiClient connections associated to a server port (like http/80).
There is a ::write() method which is supposed to send data to all currently connected client, that we don't implement.
Maybe I didn't understand your comment.

[JAndrassy]

I recently made a generic ArduinoOTA library for AVR, SAMD, nRF5, esp8266, esp32 for many different networking libraries (for example to upload to esp8266 over Ethernet). The code of the library interacts with client object as Client class. And there is a server to receive the binary, but there was no use for Server base class. it has only begin(), but can't have available() because the return type of available() is the class derived from Client class. So there is no real use of the Server class as base class. I used C++ template to work with actual server class (ArduinoOTA.h)

Now I write a new WiFiEsp library (Arduino WiFi API over AT commands of connected esp8266). I decided not to derive the WiFiServer class from Server class in this library. Then I don't need to add dummy write(b) and can add optional parameters to begin(). And all use cases are still supported, except of the never used "write to all clients" feature.

TL;DR Arduino API Server class is useles