Update docs

docs/sophia_features.md

@@ -482,6 +482,25 @@ function g(p : int * option(int)) : int =
   b
 ```
 
+Guards are boolean functions that can be used on patterns in both switch
+statements and functions definitions:
+
+```sophia
+function get_left_if_positive(x : one_or_both('a, 'b)) : option('a) =
+  switch(x)
+    Left(x)    | x > 0 => Some(x)
+    Both(x, _) | x > 0 => Some(x)
+    _                  => None
+```
+
+```sophia
+function
+  get_left_if_positive : one_or_both('a, 'b) => option('a)
+  get_left(Left(x))    | x > 0 = Some(x)
+  get_left(Both(x, _)) | x > 0 = Some(x)
+  get_left(_)                  = None
+```
+
 *NOTE: Data types cannot currently be recursive.*
 
 ## Lists