HADOOP-13698. Document caveat for KeyShell when underlying KeyProvider does not delete a key.

hadoop-common-project/hadoop-common/src/site/markdown/CommandsManual.md

@@ -202,7 +202,9 @@ Manage keys via the KeyProvider. For details on KeyProviders, see the [Transpare
 
 Providers frequently require that a password or other secret is supplied. If the provider requires a password and is unable to find one, it will use a default password and emit a warning message that the default password is being used. If the `-strict` flag is supplied, the warning message becomes an error message and the command returns immediately with an error status.
 
-NOTE: Some KeyProviders (e.g. org.apache.hadoop.crypto.key.JavaKeyStoreProvider) does not support uppercase key names.
+NOTE: Some KeyProviders (e.g. org.apache.hadoop.crypto.key.JavaKeyStoreProvider) do not support uppercase key names.
+
+NOTE: Some KeyProviders do not directly execute a key deletion (e.g. performs a soft-delete instead, or delay the actual deletion, to prevent mistake). In these cases, one may encounter errors when creating/deleting a key with the same name after deleting it. Please check the underlying KeyProvider for details.
 
 ### `trace`