Clarify behaviour of failing gaurds (#7334)

[ci skip]

Author: michalmuskala

lib/elixir/pages/Guards.md

@@ -109,7 +109,8 @@ Other constructs are `for`, `with`, `try`/`rescue`/`catch`/`else`/, and the `mat
 
 ## Failing guards
 
-Errors in guards do not result in runtime errors, but in guards failing. For example, the `length/1` function only works with lists. If we use it with anything else, a runtime error is raised:
+In guards, when functions would normally raise exceptions, they cause the guard to fail instead.
+For example, the `length/1` function only works with lists. If we use it with anything else, a runtime error is raised:
 
 ```elixir
 iex> length("hello")