[Documentation] Added another example in `df.clip` documentation.
df.clip takes lower and upper threshholds as inputs, and both of these can be `float or array-like, default None` type. 

I am aware of an example in doc which Clips specific lower and upper thresholds per column element. 

But it would be also nice to have example clipping is happening per column with specific lower and upper threshhold. 
Thanks @abhi-glitchhg 