Doc multipart set_content, tweak cte discussion.
Clarify what raw_data_manager provides.

The fact that it operates on Message objects makes the name a
little less appropriate, but conceptually its rationale is that
it provides the minimum layer on top of the existing facilities,
which also handle Message objects.  So say that.
Clarify message/partial handling; wrap long line.
Add 'charset' to set for str, reorder set prototypes.
Document that we convert to MIME charset aliases.

Note that the new code drops the "translation" from "input charset" to
"output charset":  since the input is only unicode in this API, there
can be no translation, only encoding to the target character set.  (In
Python3 I don't really see a reason to support such translation in
the general case, either.)
raw_data_manager set_text_content.