Expand table for CCS capture rates

On #28, @volker-krey wrote in [this review](https://github.com/iiasa/message_doc/pull/28#pullrequestreview-277581031) (lightly edited and emphasis added).

> [U]sing the example of coal methanol-to-gasoline with CCS (meth_coal_ccs in AFR for 2030)[…] I can reproduce the 85% capture rate for the residual emissions, i.e. the emissions excluding the carbon that remains in the fuel produced. My suggestion would be to **provide two capture rates with a clear definition in this table**:
>
> i. the current capture rate as a fraction of the process CO2 emissions and
> ii. the capture rate relative to the carbon content of the feedstock.
>
> For the above example, the latter is about 55% as a significant share of carbon remains in the fuel. The former number is interesting from an engineering point of view and describes the efficiency of the capture process. The latter number is particularly relevant for the biomass-based CDR technologies as it describes the potential to sequester carbon from the atmosphere. Obviously for processes that produce carbon-free energy carriers like electricity of hydrogen the two are the same.

That PR was assigned to @OFR-IIASA, but I leave this follow-up issue unassigned.

Start by reviewing PR #28 and the linked review, then locate the table describing CCS capture rates in the documentation. Update the table to distinguish capture as a fraction of process CO2 emissions from capture relative to feedstock carbon content, using the meth_coal_ccs example in AFR for 2030 as a check. Done means both rates have clear definitions and the table covers the stated cases.

Written by the indexing model from the issue text.