Rework if/2 docs and mention variable scope (#14452)

Author: sabiwara

lib/elixir/lib/kernel.ex

@@ -3898,51 +3898,70 @@ defmodule Kernel do
 
   ## One-liner examples
 
-      if(foo, do: bar)
+      iex> if 7 > 5, do: "yes"
+      "yes"
 
-  In the example above, `bar` will be returned if `foo` evaluates to
-  a truthy value (neither `false` nor `nil`). Otherwise, `nil` will be
-  returned.
+      iex> if "truthy value", do: "yes"
+      "yes"
+
+  In the examples above, the `do` clause is evaluated and `"yes"` will be returned
+  because the condition evaluates to a truthy value (neither `false` nor `nil`).
+  Otherwise, the clause is not evaluated and `nil` will be returned:
+
+      iex> if 3 > 5, do: "yes"
+      nil
+
+      iex> if nil, do: IO.puts("this won't be printed")
+      nil
 
   An `else` option can be given to specify the opposite:
 
-      if(foo, do: bar, else: baz)
+      iex> if 3 > 5, do: "yes", else: "no"
+      "no"
 
   ## Blocks examples
 
   It's also possible to pass a block to the `if/2` macro. The first
   example above would be translated to:
 
-      if foo do
-        bar
-      end
+      iex> if 7 > 5 do
+      ...>   "yes"
+      ...> end
+      "yes"
 
-  Note that `do`-`end` become delimiters. The second example would
+  Note that `do`-`end` become delimiters. The third example would
   translate to:
 
-      if foo do
-        bar
-      else
-        baz
-      end
-
-  ## Examples
-
-      iex> if 5 > 7 do
-      ...>   "This will never be returned"
+      iex> if 3 > 5 do
+      ...>   "yes"
       ...> else
-      ...>   "This will be returned"
+      ...>   "no"
       ...> end
-      "This will be returned"
-
-      iex> if 2 + 2 == 4, do: :correct
-      :correct
-
-      iex> if 2 + 2 == 5, do: :correct
-      nil
+      "no"
 
   If you find yourself nesting conditionals inside conditionals,
   consider using `cond/1`.
+
+  ## Variables scope
+
+  Variables set within `do`/`else` blocks do not leak to the outer context:
+
+      x = 1
+      if x > 0 do
+        x = x + 1
+        IO.puts(x)  # prints 2
+      end
+      x  # 1
+
+  Variables set in the condition are available in the outer context:
+
+      fruits = %{oranges: 3}
+      if count = fruits[:apples] do
+        # won't be evaluated
+        IO.puts(count + 1)
+      end
+      count  # nil
+
   """
   defmacro if(condition, clauses) do
     build_if(condition, clauses)