docs: embed: add parameters to docstring

The docstring of IPython.terminal.embed.embed() does not document its
arguments. Add a "Parameters" section, and document each argument
appropriately:

- for 'header', rephrase the corresponding description from the
docstring of InteractiveShellEmbed.__call__, to which this argument is
passed (__call__ itself is missing from the generated API documentation
since sphinx.ext.autodoc does not document special members by default).
- for 'compile_flags', refer to the documentation of
InteractiveShellEmbed.mainloop, to which this argument is passed.
- for 'kwargs', refer to the constructor of InteractiveShellEmbed, and
fold in the sentence mentioning the 'config' argument.

IPython/terminal/embed.py

@@ -369,8 +369,18 b' def embed(*, header="", compile_flags=None, **kwargs):'
         d = 40
         embed()
 
-    Full customization can be done by passing a traitlets :class:`Config` in as the
-    `config` argument (see :ref:`configure_start_ipython`).
+    Parameters
+    ----------
+
+    header : str
+        Optional header string to print at startup.
+    compile_flags
+        Passed to the `compile_flags` parameter of :py:meth:`InteractiveShellEmbed.mainloop()`,
+        which is called when the :class:`InteractiveShellEmbed` instance is called.
+    **kwargs : various, optional
+        Any other kwargs will be passed to the :class:`InteractiveShellEmbed` constructor.
+        Full customization can be done by passing a traitlets :class:`Config` in as the
+        `config` argument (see :ref:`configure_start_ipython`).
     """
     config = kwargs.get('config')
     if config is None: