bpo-25810: Clarify eval() docs, it does not keywords (GH-15173)

diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 2e385a2..ea2777f 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -456,7 +456,7 @@
               n += 1
 
 
-.. function:: eval(expression, globals=None, locals=None)
+.. function:: eval(expression[, globals[, locals]])
 
    The arguments are a string and optional globals and locals.  If provided,
    *globals* must be a dictionary.  If provided, *locals* can be any mapping