rose_prune: improve docs

[oliver-sanders]

Following on from the recent training session, clarify some things in the rose_prune docs.

Notes:

• The rose_prune app was only ever implemented for Cylc workflows, there is no plan to extend this to support other workflow engines.
• The rose_prune app must be run on the local filesystem.
• The prune{item-root} setting removes files from local and remote platforms.

To see what files rose_prune will remove with the prune{item-root} config, use this diff:

diff --git a/metomi/rose/apps/rose_prune.py b/metomi/rose/apps/rose_prune.py
index 43ef7b5a8..87c51faec 100644
--- a/metomi/rose/apps/rose_prune.py
+++ b/metomi/rose/apps/rose_prune.py
@@ -296,4 +296,5 @@ class RosePruneApp(BuiltinApp):
                         globs.append(os.path.join(head, glob_))
                 else:
                     globs.append(os.path.join(head, cycle))
+        print('#', globs)
         return globs, cycle_set