Update device removal documentation

[behlendorf]

Motivation and Context

The man page was incomplete and the documentation in the code was getting increasingly stale. Fix it.

Description

Make a minor update to the 'zpool remove' man page to clarify both raidz and draid pools do not support removal, and change sector to ashift which is what we actually care about.

Update the big theory comment in vdev_removal.c to accurately reflect which types of vdevs can be removed. Furthermore, I've added some discussion for the casual reader to briefly explain the top-level vdev removal restrictions. This has been a common area of confusion and it's not intuitive where they come from without understanding the implementation details.

How Has This Been Tested?

Eyeballed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.