docs(canvas): add support note for Braille marker (#472)

src/symbols.rs

@@ -244,14 +244,21 @@ pub mod braille {
 /// Marker to use when plotting data points
 #[derive(Debug, Default, Display, EnumString, Clone, Copy, Eq, PartialEq, Hash)]
 pub enum Marker {
-    /// One point per cell in shape of dot
+    /// One point per cell in shape of dot ("•")
     #[default]
     Dot,
-    /// One point per cell in shape of a block
+    /// One point per cell in shape of a block ("█")
     Block,
-    /// One point per cell in the shape of a bar
+    /// One point per cell in the shape of a bar ("▄")
     Bar,
-    /// Up to 8 points per cell
+    /// Use the [Unicode Braille Patterns](https://en.wikipedia.org/wiki/Braille_Patterns) block to
+    /// represent data points.
+    ///
+    /// This is a 2x4 grid of dots, where each dot can be either on or off.
+    ///
+    /// Note: Support for this marker is limited to terminals and fonts that support Unicode
+    /// Braille Patterns. If your terminal does not support this, you will see unicode replacement
+    /// characters (<28>) instead of Braille dots.
     Braille,
 }
 

src/widgets/canvas/mod.rs

@@ -320,8 +320,16 @@ impl<'a> Context<'a> {
     }
 }
 
-/// The Canvas widget may be used to draw more detailed figures using braille patterns (each
+/// The Canvas widget provides a means to draw shapes (Lines, Rectangles, Circles, etc.) on a grid.
-/// cell can have a braille character in 8 different positions).
+///
+/// By default the grid is made of Braille patterns but you may change the marker to use a different
+/// set of symbols. If your terminal or font does not support this unicode block, you will see
+/// unicode replacement characters (<28>) instead of braille dots. The Braille patterns provide a more
+/// fine grained result (2x4 dots) but you might want to use a simple dot, block, or bar instead by
+/// calling the [`marker`] method if your target environment does not support those symbols,
+///
+/// See [Unicode Braille Patterns](https://en.wikipedia.org/wiki/Braille_Patterns) for more info.
+///
 /// # Examples
 ///
 /// ```
@@ -355,6 +363,8 @@ impl<'a> Context<'a> {
 ///         });
 ///     });
 /// ```
+///
+/// [`marker`]: #method.marker
 #[derive(Debug, Clone, PartialEq)]
 pub struct Canvas<'a, F>
 where