Fixed documentation about use of salt parameter in signing functions.

Using the salt argument

-----------------------

If you do not wish to use the same key for every signing operation in your

application, you can use the optional ``salt`` argument to the ``Signer``

class to further strengthen your :setting:`SECRET_KEY` against brute force

attacks. Using a salt will cause a new key to be derived from both the salt

and your :setting:`SECRET_KEY`::

If you do not wish for every occurrence of a particular string to have the same

signature hash, you can use the optional ``salt`` argument to the ``Signer``

class. Using a salt will seed the signing hash function with both the salt and

your :setting:`SECRET_KEY`::

    >>> signer = Signer()

    >>> signer.sign('My string')

    >>> signer.unsign('My string:Ee7vGi-ING6n02gkcJ-QLHg6vFw')

    u'My string'

Using salt in this way puts the different signatures into different

namespaces.  A signature that comes from one namespace (a particular salt

value) cannot be used to validate the same plaintext string in a different

namespace that is using a different salt setting. The result is to prevent an

attacker from using a signed string generated in one place in the code as input

to another piece of code that is generating (and verifying) signatures using a

different salt.

Unlike your :setting:`SECRET_KEY`, your salt argument does not need to stay

secret.