Fix spanner API documentation

[vkedia]

@lukesneeringer @stephenplusplus @callmehiphop
I was going through the spanner API documentation and I noticed various issues. Some of which I am enumerating below but we need to go thoroughly over the documentation and make sure it is accurate and complete:

• In transaction, the link to database/runTransaction does not work.
• Under parameters for database/runTransaction we should document that options like exactStaleness are only relevant for readOnly transactions.
• database/runTransaction does not mention that it retries the runFunction on ABORT errors.
• In transaction, there is a table of Parameters. But it is not clear where those parameters are to be specified. I think that table can be removed from there. There already is a similar table in runTransaction.
• The table mentioned above, talks about single-use transaction but nowhere is that term defined and it is not clear how to use single-use transaction in the node client.
• In transaction/deleteRows, Parameters talk about a callback but that method does not take a callback. Ditto for replace, update and upsert.
• Second argument for insert/update/replace/upsert is called keyVals and it says that it is "A map of names to values of data to insert into this table". That seems incorrect. It is actually a list of rows where a row is a map from column name to column value.
• database/run and other similar methods have a parameter, returnReadTimestamp which is true returns the timestamp at which the data was read to the user. But it is not clear how it is returned in example or the documentation.