docs: add default threads count about NioEventLoopGroup

[brido4125]

add NioEventLoopGroup default threads counts in AbstractRedisClient javadoc.

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[tishun]

Hey @brido4125 ,
can we take a step back and discuss the issue.

I assume you found it hard to understand the defaults, correct? Would it make sense to add this to the documentation instead (not sure how many users dig in the source code to that level) ?