Document missing API features across guides

- chords: open_voicing() alongside other voicings, normal_form() in
  pitch class sets section
- tones: is_natural, is_sharp, is_flat accidental properties
- scales: Key.seventh() for individual degrees, expanded
  Scale.recommend() explanation of how ranking works

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/guide/chords.rst

@@ -503,9 +503,16 @@ are standard arranging techniques for spreading chord tones across registers:
    >>> cmaj7 = Chord.from_symbol("Cmaj7")
    >>> cmaj7.close_voicing()
    <Chord C major 7th>
+   >>> cmaj7.open_voicing()
+   <Chord C major 7th>
    >>> cmaj7.drop2()
    <Chord C major 7th>
 
+``open_voicing()`` takes the close voicing and raises every other
+non-root tone by an octave, spreading the chord across two octaves.
+The result is a wider, more spacious sound — common in orchestral
+writing and piano ballads where you want the harmony to breathe.
+
 Chord Extensions
 ----------------
 
@@ -596,6 +603,23 @@ music that doesn't follow traditional harmony, this is the tool.
 Major and minor triads share the same prime form — they're inversions
 of each other in pitch class space.
 
+The **normal form** is the intermediate step — the most compact ascending
+arrangement of pitch classes before transposition. It preserves the
+actual pitch classes (not transposed to 0), so it tells you which
+specific notes are in the set:
+
+.. code-block:: pycon
+
+   >>> Chord.from_tones("C", "E", "G").normal_form
+   (0, 4, 7)
+
+   >>> Chord.from_tones("A", "C", "E").normal_form
+   (9, 0, 4)
+
+Normal form keeps the original pitch classes; prime form transposes to 0
+for comparison. Use ``normal_form`` when you care about which notes,
+``prime_form`` when you care about the abstract shape.
+
 .. code-block:: pycon
 
    >>> Chord.from_tones("C", "E", "G").forte_number

docs/guide/scales.rst

@@ -269,6 +269,23 @@ easy:
    ['G major', 'A minor', 'B minor', 'C major', 'D major', 'E minor', 'F# diminished']
    >>> key.seventh_chords
    ['G major 7th', 'A minor 7th', 'B minor 7th', 'C major 7th', 'D dominant 7th', 'E minor 7th', 'F# half-diminished 7th']
+
+Build a seventh chord on any individual degree with ``seventh()``:
+
+.. code-block:: pycon
+
+   >>> key.seventh(0)   # I7
+   G major 7th
+   >>> key.seventh(4)   # V7
+   D dominant 7th
+   >>> key.seventh(6)   # vii7
+   F# half-diminished 7th
+
+This is the single-degree version of ``seventh_chords`` — useful when
+you need one specific chord rather than the full list.
+
+.. code-block:: pycon
+
    >>> Key.detect("C", "E", "G", "A", "D")
    C major
 
@@ -440,7 +457,16 @@ alternative scales to improvise over:
    >>> Scale.recommend("C", "Eb", "F", "Gb", "G", "Bb", top=3)
    [('C', 'blues', 1.0), ...]
 
-Chromatic scales are deprioritized since they match everything.
+How it works: ``recommend()`` tests your notes against every scale in
+every key (all 12 tonics times all scale types in the Western system).
+Each candidate is scored using ``fitness()`` — the fraction of your notes
+that belong to that scale (1.0 = perfect match). Results are ranked by
+fitness, with chromatic scales deprioritized since they match everything.
+Scales whose length is closer to the number of input notes are preferred
+when fitness scores tie.
+
+Returns a list of ``(tonic, scale_name, fitness)`` tuples. Pass ``top=``
+to control how many results you get back (default 5).
 
 Parallel Modes
 ~~~~~~~~~~~~~~

docs/guide/tones.rst

@@ -357,6 +357,32 @@ every tone knows its enharmonic spelling:
    >>> Tone.from_string("C4", system="western").enharmonic is None
    True
 
+Accidental Properties
+~~~~~~~~~~~~~~~~~~~~~
+
+Check whether a tone is natural, sharp, or flat:
+
+.. code-block:: pycon
+
+   >>> c = Tone.from_string("C4", system="western")
+   >>> c.is_natural
+   True
+   >>> c.is_sharp
+   False
+
+   >>> cs = Tone.from_string("C#4", system="western")
+   >>> cs.is_sharp
+   True
+   >>> cs.is_natural
+   False
+
+   >>> bb = Tone.from_string("Bb4", system="western")
+   >>> bb.is_flat
+   True
+
+Useful for filtering — for example, finding all natural notes in a
+scale, or counting accidentals in a melody.
+
 Extended Enharmonics
 ~~~~~~~~~~~~~~~~~~~~