Encoding Object content and header clarifications (3.2.0 port of 3857 1/4)

The Encoding Object's `contentType` field takes a comma-separated
list of either regular or wildcared media types.  These are the
"two types" mentioned in the previous wording – "two" here did
*not* refer to a limit on the number of entries in the list.

These are not exactly media-type or media-range values, as both
of those include parameters.

This change also moves the hard-to-follow list of default values
out of the individual field cell and into its own table.

It takes `Content-Disposition` out of the header field's cell
and instead explains limitations on header usage, and explains
how `Content-Disposition` is used for encoding.

This explanation includes a suggestion on how other `multipart`
formats could be used with an Encoding Object, since their
unnamed parts otherwise cannot be supported.

Finally, it clarifies the interaction between `contentType`
and the three fields imported from the Parameter Object, by
aligning the recommended (but not, for compatibility reasons,
required) behavior with guidance added in 3.1.0.