add description for MaximumClientWritesPending

[CendealChen]

pls see ref MaximumClientWritesPending建议添加文档说明其意义及影响 #410

[thedevop]

You may also want to point it out that setting this value very low may result in lost messages for subscribers, especially for busy topics. One can monitor the metrics MessagesDropped when that occurred.

I do agree the current default is biased towards not dropping messages, perhaps 128 may be sufficient, which translates to a fixed memory cost of 1KB per connection. That makes it smaller than ClientNetReadBufferSize and ClientNetWriteBufferSize, both at 2KB.

[mochi-co]

Always happy to have more extensive documentation, thank you @CendealChen