[Documentation -> Swapfile]: Add Sections »Removal« and »Resizing«

This is a Request for adding the following Information to [This Documentation](https://btrfs.readthedocs.io/en/latest/Swapfile.html):

+ **Swapfile Removal**
+ **Swapfile Resizing**

Basically the steps to be done to remove or resize a swapfile in a BTRFS filesystem properly.