[Sass] Fix a little documentation for the stack-handling additions.

lib/sass/environment.rb

@@ -63,13 +63,19 @@ module Sass
 
     # A list of the runtime stack frame information
     # The last element in the list was pushed onto the stack most recently.
+    #
+    # @return [Array<{Symbol => Object}>] The stack frames,
+    #   of the form passed to \{#push}.
     def stack
       prev = (!@ignore_parent_stack && parent && parent.stack) || []
       prev + @stack
     end
 
     # Temporarily assume the runtime stack that is passed in.
-    # @param stk A stack value from another environment.
+    #
+    # @param stk [Array<{Symbol => Object}>] A call stack from another environment,
+    #   of the form returned by \{#stack}.
+    # @yield A block in which the stack is set to `stk`.
     def with_stack(stk)
       @stack, old_stack = stk, @stack
       @ignore_parent_stack = true