docs: add reana-client retention-rules-show command

Closes reanahub/reana-client#620

docs/reference/reana-client-cli-api/index.md

@@ -49,6 +49,9 @@ Workspace file management commands:
   rm        Delete files from workspace.
   upload    Upload files and directories to workspace.
 
+Workspace file retention commands:
+  retention-rules-show  Show the retention rules of a workflow.
+
 Secret management commands:
   secrets-add     Add secrets from literal string or from file.
   secrets-delete  Delete user secrets by name.
@@ -401,6 +404,16 @@ Examples:
 
      $ reana-client du -w myanalysis.42 --filter name=data/
 
+## Workspace file retention commands
+
+### retention-rules-show
+
+Show the retention rules of a workflow.
+
+Example:
+
+     $ reana-client retention-rules-show -w myanalysis.42
+
 ## Secret management commands
 
 ### secrets-add