docs: lshelp: explain that a directory on a bucket-based remote is a key prefix

Listing a bucket without -R shows each prefix as one directory entry, so the
objects under it are not in the output. This is easy to read as a truncated
listing rather than as one level of a hierarchy.

Fixes #9797

cmd/ls/lshelp/lshelp.go

@@ -26,6 +26,10 @@ Note that |ls| and |lsl| recurse by default - use |--max-depth 1| to stop the re
 The other list commands |lsd|,|lsf|,|lsjson| do not recurse by default -
 use |-R| to make them recurse.
 
+On the bucket-based remotes (e.g. s3, gcs, b2) a directory is a key prefix
+rather than an object, so a listing without |-R| shows each prefix as one
+directory entry and not the objects stored under it.
+
 List commands prefer a recursive method that uses more memory but fewer
 transactions by default. Use |--disable ListR| to suppress the behavior.
 See [|--fast-list|](/docs/#fast-list) for more details.