Remove requirements in description of `volume-modify` commands

[vickmuir]

Describe the bug
The limitations around the 0.25 tier of Endurance and 0.3 IOPS/GB performance level of Performance storage have been resolved by replacing the HDD storage with SSDs in all our DCs. So when our customers want to modify the IOPS of their storage volumes, they can do so without the previous restrictions.

The description of the following commands contains requirements that can be removed:

• ibmcloud sl file volume-modify
• ibmcloud sl block volume-modify
• slcli file volume-modify
• slcli block volume-modify

Expected behavior
CLI allows the customers to increase their IOPS without increasing the volume size even if they originally provisioned their volume in the lowest IOPS tier. This issue is created to update the description and remove the requirements that are presented when the customer uses -help to see how the command is to be used.

Screenshots

Version
Include the output of slcli --version

[allmightyspiff]

Updated block|file volume-modify help message