Correct workspace doc on behaviour of mountPath
Correct workspace doc on behaviour of mountPath

Prior to this commit we documented that if a workspace's mountPath was
relative then Pipelines would prepend "/workspace/" to it. This behaviour
has never actually worked that way. Pipelines passes the literal mountPath
value directly to the volumeMount's mountPath, without any other
processing. It's then up to the platform to decide what to do with it.

This commit fixes the workspaces doc to remove the incorrect description.

tektoncd/pipeline,tektoncd/pipeline