Update defstruct docs with mention of msgspec.field

Also adds an example.

msgspec/_core.c

@@ -6209,8 +6209,9 @@ PyDoc_STRVAR(msgspec_defstruct__doc__,
 "    The name of the new Struct class.\n"
 "fields : iterable\n"
 "    An iterable of fields in the new class. Elements may be either ``name``,\n"
-"    tuples of ``(name, type)``, or ``(name, type, default)``. Fields without\n"
-"    a specified type will default to ``typing.Any``.\n"
+"    tuples of ``(name, type)``, ``(name, type, default)``, or \n"
+"    ``(name, type, msgspec.field)``. Fields without a specified type will \n"
+"    default to ``typing.Any``.\n"
 "bases : tuple, optional\n"
 "    A tuple of any Struct base classes to use when defining the new class.\n"
 "module : str, optional\n"
@@ -6223,6 +6224,20 @@ PyDoc_STRVAR(msgspec_defstruct__doc__,
 "    Additional Struct configuration options. See the ``Struct`` docs for more\n"
 "    information.\n"
 "\n"
+"Examples\n"
+"--------\n"
+">>> from msgspec import defstruct, field\n"
+">>> User = defstruct(\n"
+"...     'User',\n"
+"...     [\n"
+"...         ('name', str),\n"
+"...         ('email', str | None, None),\n"
+"...         ('groups', set[str], field(default_factory=set)),\n"
+"...     ],\n"
+"... )\n"
+">>> User('alice')\n"
+"User(name='alice', email=None, groups=set())\n"
+"\n"
 "See Also\n"
 "--------\n"
 "Struct"