Write docs for Shaping enum

src/shape.rs

@@ -11,9 +11,22 @@ use unicode_segmentation::UnicodeSegmentation;
 use crate::fallback::FontFallbackIter;
 use crate::{Align, AttrsList, CacheKey, Color, Font, FontSystem, LayoutGlyph, LayoutLine, Wrap};
 
+/// The shaping strategy of some text.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum Shaping {
+    /// Basic shaping with no font fallback.
+    ///
+    /// This shaping strategy is very cheap, but it will not display complex
+    /// scripts properly nor try to find missing glyphs in your system fonts.
+    ///
+    /// You should use this strategy when you have complete control of the text
+    /// and the font you are displaying in your application.
     Basic,
+    /// Advanced text shaping and font fallback.
+    ///
+    /// You will need to enable this strategy if the text contains a complex
+    /// script, the font used needs it, and/or multiple fonts in your system
+    /// may be needed to display all of the glyphs.
     Advanced,
 }