DOC: Add composition example to internals.rst
Follow-up of #9802. Added an example of composition to internal.rst. 

Also, fixed broken section name for subclass document.

I made a few grammar/language suggestions.

But actually, I'm not sure this example makes the best case for why composition is usually a better idea than subclassing. Using `__getattr__` makes this `ComposedSeries` example a sort of limited version of subclassing.

So many we should show an example of how subclassing can lead to unexpected behavior or a more realistic version of composition that simply makes use of a series or frame to hold internal state.
 status?
 @sinhrks going to move this to next version.
 @sinhrks can you update
 closing, but if you'd like to update, pls reopen
