docs/guide: Remove obsolete warning about undefined FSM states

[neuschaefer]

>>> m = Module()
>>> with m.FSM():
...     with m.State("Init"):
...         m.next = "Other"
...
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    with m.FSM():
         ~~~~~^^
  File "/usr/lib/python3.14/contextlib.py", line 148, in __exit__
    next(self.gen)
    ~~~~^^^^^^^^^^
  File "/.../amaranth/amaranth/hdl/_dsl.py", line 480, in FSM
    raise NameError("FSM state '{}' is referenced but not defined"
                    .format(state_name))
NameError: FSM state 'Other' is referenced but not defined
>>>

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (56f1cab) to head (94368a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1684   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files          44       44           
  Lines       11527    11527           
  Branches     2245     2245           
=======================================
  Hits        10519    10519           
  Misses        846      846           
  Partials      162      162           

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[whitequark]

Thanks!