docs: Issue in Client-side Timestamps

[fee-mendes]

In https://raw.githubusercontent.com/scylladb/cpp-driver/master/topics/basics/client_side_timestamps/README.md we say:

(...) By default, timestamps are assigned by the driver on the client-side. This behavior can be overridden by configuring the driver to use a timestamp generator or assigning a timestamp directly to a CassStatement or CassBatch.

It is not clear exactly what this means. In particular, if the default behavior is to assign timestamps on the client-side, then overriding that behavior would be basically NOT using a timestamp generator or - of course - assigning a timestamp manually to an statement.

Here's the same sentence from Datastax CPP docs:

By default, timestamps are assigned by Cassandra on the server-side. This behavior can be overridden by configuring the driver to use a timestamp generator or assigning a timestamp directly to a CassStatement or CassBatch.

In this case the sentence is semantically correct.

@annastuchlik