Improve ResultProxy/Row documentation before the API change

Improve ResultProxy/Row documentation before the API change

We'd like to merge Ieb9085e9bcff564359095b754da9ae0af55679f0,
however the documentation in that change should be based off
of more comprehensive documentation than what we have already.

Add the notion of "row" to the tutorial and document all
methods.   This will also be backported at least to 1.3
in terms of RowProxy.

Change-Id: I2173aecc86cf15c5a7c473b8f471639ee9082f84