Add warning for chat messages too long in docs

[double-fault]

Add a warning to the docs to alert the user that long chat messages will automatically be ignored, without an error (that's what CE does).

[tripleee]

How long are messages which are too long?

[double-fault]

@tripleee Looks like it is 500 characters. Multi-line messages don't have a character limit, so you have to manually disable the length_check option.

[jdd-software]

Also Tuna is using 500, this is the code he used (instead of warning it split the message into multiple messages) https://github.com/SOBotics/chatexchange/blob/develop/src/main/java/org/sobotics/chatexchange/chat/Room.java#L262