Details on commonly expected error codes need to be added to the generated docs

[vsubramani]

For each method, the generated docs currently says that a GaxError is raised if the RPC is aborted. It should instead document:

a) What GAX errors are commonly raised by this method
b) What is the underlying meaning of the error

Also, consider adding a common page for aggregating all types of GAX errors which provide the information on:

a) What GAX errors exist
b) What is the underlying meaning of each error.

/cc @rok987
/cc @andrewmacvean
/cc @tbetbetbe

[bjwatson]

This is really a https://github.com/googleapis/toolkit or https://github.com/googleapis/packman issue. We should decide which one, and create an issue there to track this.