[Braintree] Change description for createBraintreeClientToken mutation

[YevSent]

The createBraintreeClientToken mutation sends a request to generate a client token used by JS SDK for initialization. The current description says: Creates Braintree Client Token for creating client-side nonce. but actually this client token needed to initialize SDK but not to generate a payment nonce. Also, it might be used in different scenarios like Kount initialization.

Proposed solution

Change the mutation description to Creates Client Token for Braintree Javascript SDK initialization..

[karyna-t]

I'm going to work on this issue.

[lenaorobei]

#959