add some documentation notes about trust being per-profile

IPython/nbformat/sign.py

@@ -263,10 +263,17 class TrustNotebookApp(BaseIPythonApplication):
     description="""Sign one or more IPython notebooks with your key,
     to trust their dynamic (HTML, Javascript) output.
     
+    Trusting a notebook only applies to the current IPython profile.
+    To trust a notebook for use with a profile other than default,
+    add `--profile [profile name]`.
+    
     Otherwise, you will have to re-execute the notebook to see output.
     """
     
-    examples = """ipython trust mynotebook.ipynb and_this_one.ipynb"""
+    examples = """
+    ipython trust mynotebook.ipynb and_this_one.ipynb
+    ipython trust --profile myprofile mynotebook.ipynb
+    """
     
     flags = trust_flags
     

docs/source/notebook/security.rst

@@ -49,6 +49,12 the IPython profile's security directory. By default, this is::
 
     ~/.ipython/profile_default/security/notebook_secret
 
+.. note::
+
+    The notebook secret being stored in the profile means that
+    loading a notebook in another profile results in it being untrusted,
+    unless you copy or symlink the notebook secret to share it across profiles.
+
 When a notebook is opened by a user, the server computes a signature
 with the user's key, and compares it with the signature stored in the
 notebook's metadata. If the signature matches, HTML and Javascript