g_base64_encode_step: clarify break_lines behavior a bit
g_base64_encode_step: clarify break_lines behavior a bit

break_lines uses LFs, not CRLFs like you might expect (since it's
designed for email-related use), but we can't change that now since
the caller has to allocate the output buffer and so the
number-of-bytes-output is part of the ABI. So, just document that.

https://bugzilla.gnome.org/show_bug.cgi?id=668158

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