Feval_buffer doc string lexical-binding clarification

* src/lread.c (Feval_buffer): Mention that the lexical-binding
variable is ignored in the doc string (bug#20139).

src/lread.c

@@ -2147,6 +2147,12 @@ DO-ALLOW-PRINT, if non-nil, specifies that output functions in the
  evaluated code should work normally even if PRINTFLAG is nil, in
  which case the output is displayed in the echo area.
 
+This function ignores the current value of the `lexical-binding'
+variable.  Instead it will heed any
+  -*- lexical-binding: t -*-
+settings in the buffer, and if there is no such setting, the buffer
+will be evaluated without lexical binding.
+
 This function preserves the position of point.  */)
   (Lisp_Object buffer, Lisp_Object printflag, Lisp_Object filename, Lisp_Object unibyte, Lisp_Object do_allow_print)
 {