docs: Add some details about coordinates

Mention what the effective values are when
coordinates have not been set.

src/hb-font.cc

@@ -631,7 +631,7 @@ hb_font_funcs_destroy (hb_font_funcs_t *ffuncs)
  * @destroy: (nullable): A callback to call when @data is not needed anymore
  * @replace: Whether to replace an existing data with the same key
  *
- * Attaches a user-data key/data pair to the specified font-functions structure. 
+ * Attaches a user-data key/data pair to the specified font-functions structure.
  *
  * Return value: %true if success, %false otherwise
  *
@@ -821,7 +821,7 @@ hb_font_get_glyph (hb_font_t      *font,
  * @glyph: (out): The glyph ID retrieved
  *
  * Fetches the nominal glyph ID for a Unicode code point in the
- * specified font. 
+ * specified font.
  *
  * This version of the function should not be used to fetch glyph IDs
  * for code points modified by variation selectors. For variation-selector
@@ -940,7 +940,7 @@ hb_font_get_glyph_v_advance (hb_font_t      *font,
  * @advance_stride: The stride between successive advances
  *
  * Fetches the advances for a sequence of glyph IDs in the specified
- * font, for horizontal text segments. 
+ * font, for horizontal text segments.
  *
  * Since: 1.8.6
  **/
@@ -964,7 +964,7 @@ hb_font_get_glyph_h_advances (hb_font_t*            font,
  * @advance_stride: (out): The stride between successive advances
  *
  * Fetches the advances for a sequence of glyph IDs in the specified
- * font, for vertical text segments.  
+ * font, for vertical text segments.
  *
  * Since: 1.8.6
  **/
@@ -1278,7 +1278,7 @@ hb_font_get_glyph_origin_for_direction (hb_font_t      *font,
  * @font: #hb_font_t to work upon
  * @glyph: The glyph ID to query
  * @direction: The direction of the text segment
- * @x: (inout): Input = The original X coordinate 
+ * @x: (inout): Input = The original X coordinate
  *     Output = The X coordinate plus the X-coordinate of the origin
  * @y: (inout): Input = The original Y coordinate
  *     Output = The Y coordinate plus the Y-coordinate of the origin
@@ -1306,7 +1306,7 @@ hb_font_add_glyph_origin_for_direction (hb_font_t      *font,
  * @font: #hb_font_t to work upon
  * @glyph: The glyph ID to query
  * @direction: The direction of the text segment
- * @x: (inout): Input = The original X coordinate 
+ * @x: (inout): Input = The original X coordinate
  *     Output = The X coordinate minus the X-coordinate of the origin
  * @y: (inout): Input = The original Y coordinate
  *     Output = The Y coordinate minus the Y-coordinate of the origin
@@ -1681,12 +1681,12 @@ hb_font_destroy (hb_font_t *font)
 /**
  * hb_font_set_user_data: (skip)
  * @font: #hb_font_t to work upon
- * @key: The user-data key 
+ * @key: The user-data key
  * @data: A pointer to the user data
  * @destroy: (nullable): A callback to call when @data is not needed anymore
  * @replace: Whether to replace an existing data with the same key
  *
- * Attaches a user-data key/data pair to the specified font object. 
+ * Attaches a user-data key/data pair to the specified font object.
  *
  * Return value: %true if success, %false otherwise
  *
@@ -1888,7 +1888,7 @@ hb_font_set_funcs (hb_font_t         *font,
  * @font_data: (destroy destroy) (scope notified): Data to attach to @font
  * @destroy: (nullable): The function to call when @font_data is not needed anymore
  *
- * Replaces the user data attached to a font, updating the font's 
+ * Replaces the user data attached to a font, updating the font's
  * @destroy callback.
  *
  * Since: 0.9.2
@@ -1962,7 +1962,7 @@ hb_font_get_scale (hb_font_t *font,
  * @x_ppem: Horizontal ppem value to assign
  * @y_ppem: Vertical ppem value to assign
  *
- * Sets the horizontal and vertical pixels-per-em (ppem) of a font. 
+ * Sets the horizontal and vertical pixels-per-em (ppem) of a font.
  *
  * Since: 0.9.2
  **/
@@ -1984,7 +1984,7 @@ hb_font_set_ppem (hb_font_t    *font,
  * @x_ppem: (out): Horizontal ppem value
  * @y_ppem: (out): Vertical ppem value
  *
- * Fetches the horizontal and vertical points-per-em (ppem) of a font. 
+ * Fetches the horizontal and vertical points-per-em (ppem) of a font.
  *
  * Since: 0.9.2
  **/
@@ -2257,6 +2257,9 @@ hb_font_set_var_coords_normalized (hb_font_t    *font,
  * Fetches the list of normalized variation coordinates currently
  * set on a font.
  *
+ * Note that this returned array may only contain values for some
+ * (or none) of the axes; omitted axes effectively have zero values.
+ *
  * Return value is valid as long as variation coordinates of the font
  * are not modified.
  *
@@ -2280,6 +2283,10 @@ hb_font_get_var_coords_normalized (hb_font_t    *font,
  * Return value is valid as long as variation coordinates of the font
  * are not modified.
  *
+ * Note that this returned array may only contain values for some
+ * (or none) of the axes; omitted axes effectively have their default
+ * values.
+ *
  * Return value: coordinates array
  *
  * Since: REPLACEME