Clarify with a translator comment a WordCamp Stripe string

[netweb]

String: We're sorry, but we can't accept amounts larger than %s. Please send the equivalent in USD, or break it up into several smaller payments. Feel free to email <a href='mailto:%s'>%s</a> with any questions.

A translator comment should be added to note that USD should not be translated:

Via @pento:

@webaware, @netweb: Looking at the string, I think it's supposed to stay as USD. It's for the case where a person is trying to pay a larger figure than the Stripe API supports (999,999.99, in any currency). For example, 999,999.99 Chilean Pesos is ~$700 USD, which someone could conceivably run into.

Source: https://meta.trac.wordpress.org/browser/sites/trunk/wordcamp.org/public_html/wp-content/mu-plugins/utilities/class-stripe-client.php?marks=69#L69

[iandunn]

In 7070:

Stripe Client: Add note to clarify that USD should not be translated.

Props netweb.
Fixes #3555.