Mention that some options cannot be set through %config

docs/source/config/intro.rst

@@ -1,155 +1,156 b''
 =====================================
 Introduction to IPython configuration
 =====================================
 
 .. _setting_config:
 
 Setting configurable options
 ============================
 
 Many of IPython's classes have configurable attributes (see
 :doc:`options/index` for the list). These can be
 configured in several ways.
 
 Python config files
 -------------------
 
 To create the blank config files, run::
 
     ipython profile create [profilename]
 
 If you leave out the profile name, the files will be created for the
 ``default`` profile (see :ref:`profiles`). These will typically be
 located in :file:`~/.ipython/profile_default/`, and will be named
 :file:`ipython_config.py`, :file:`ipython_notebook_config.py`, etc.
 The settings in :file:`ipython_config.py` apply to all IPython commands.
 
 The files typically start by getting the root config object::
 
     c = get_config()
 
 You can then configure class attributes like this::
 
     c.InteractiveShell.automagic = False
 
 Be careful with spelling--incorrect names will simply be ignored, with
 no error.
 
 To add to a collection which may have already been defined elsewhere,
 you can use methods like those found on lists, dicts and sets: append,
 extend, :meth:`~IPython.config.loader.LazyConfigValue.prepend` (like
 extend, but at the front), add and update (which works both for dicts
 and sets)::
 
     c.InteractiveShellApp.extensions.append('rmagic')
 
 .. versionadded:: 2.0
    list, dict and set methods for config values
 
 Example config file
 ```````````````````
 
 ::
 
     # sample ipython_config.py
     c = get_config()
 
     c.TerminalIPythonApp.display_banner = True
     c.InteractiveShellApp.log_level = 20
     c.InteractiveShellApp.extensions = [
         'myextension'
     ]
     c.InteractiveShellApp.exec_lines = [
         'import numpy',
         'import scipy'
     ]
     c.InteractiveShellApp.exec_files = [
         'mycode.py',
         'fancy.ipy'
     ]
     c.InteractiveShell.autoindent = True
     c.InteractiveShell.colors = 'LightBG'
     c.InteractiveShell.confirm_exit = False
     c.InteractiveShell.deep_reload = True
     c.InteractiveShell.editor = 'nano'
     c.InteractiveShell.xmode = 'Context'
 
     c.PromptManager.in_template  = 'In [\#]: '
     c.PromptManager.in2_template = '   .\D.: '
     c.PromptManager.out_template = 'Out[\#]: '
     c.PromptManager.justify = True
 
     c.PrefilterManager.multi_line_specials = True
 
     c.AliasManager.user_aliases = [
      ('la', 'ls -al')
     ]
 
 
 Command line arguments
 ----------------------
 
 Every configurable value can be set from the command line, using this
 syntax::
 
     ipython --ClassName.attribute=value
 
 Many frequently used options have short aliases and flags, such as
 ``--matplotlib`` (to integrate with a matplotlib GUI event loop) or
 ``--pdb`` (automatic post-mortem debugging of exceptions).
 
 To see all of these abbreviated options, run::
 
     ipython --help
     ipython notebook --help
     # etc.
 
 Options specified at the command line, in either format, override
 options set in a configuration file.
 
 The config magic
 ----------------
 
 You can also modify config from inside IPython, using a magic command::
 
     %config IPCompleter.greedy = True
 
 At present, this only affects the current session - changes you make to
-config are not saved anywhere.
+config are not saved anywhere. Also, some options are only read when
+IPython starts, so they can't be changed like this.
 
 .. _profiles:
 
 Profiles
 ========
 
 IPython can use multiple profiles, with separate configuration and
 history. By default, if you don't specify a profile, IPython always runs
 in the ``default`` profile. To use a new profile::
 
     ipython profile create foo   # create the profile foo
     ipython --profile=foo        # start IPython using the new profile
 
 Profiles are typically stored in :ref:`ipythondir`, but you can also keep
 a profile in the current working directory, for example to distribute it
 with a project. To find a profile directory on the filesystem::
 
     ipython locate profile foo
 
 .. _ipythondir:
 
 The IPython directory
 =====================
 
 IPython stores its files---config, command history and extensions---in
 the directory :file:`~/.ipython/` by default.
 
 .. envvar:: IPYTHONDIR
 
    If set, this environment variable should be the path to a directory,
    which IPython will use for user data. IPython will create it if it
    does not exist.
 
 .. option:: --ipython-dir=<path>
 
    This command line option can also be used to override the default
    IPython directory.