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RFC 3548, referenced by the base64 module's docs, has a rather different
statement on how invalid characters should be treated.
From 2.3 Interpretation of non-alphabet characters in encoded data:
 Implementations MUST reject the encoding if it contains characters
 outside the base alphabet when interpreting base encoded data, unless
 the specification referring to this document explicitly states
 otherwise. Such specifications may, as MIME does, instead state that
 characters outside the base encoding alphabet should simply be
 ignored when interpreting data ("be liberal in what you accept").
So it looks like we can safely just say that invalid characters are
ignored in the docs, as long as it's explicit, but that's probably not
what people will expect.
I'll add doc patches in a moment, and someone who's actually a developer
(i.e., not me) can decide whether they're good enough.