[DOC] Fix the description of #new_toplevel

[nobu]

new_toplevel does not define new variables that is not defined already in the binding.

require 'erb'
e = ERB.new("")
b = e.send(:new_toplevel, [:x])
p b.local_variable_get(:x) #=> NameError, local variable 'x' is not defined

Conversely, hides the variables that have already been defined.

require 'erb'
e = ERB.new("")
b = e.send(:new_toplevel, [])
p b.local_variable_get(:b) == b #=> true
b = e.send(:new_toplevel, [:b])
p b.local_variable_get(:b)      #=> nil

[BurdetteLamar]

@nobu, I'm not sure what to do here. Certainly I approve your change.