Update the Column.nullable docstring for the primary_key=True case

While reviewing a change that created a new table, the
primary_key column value was set to True but nullable was not
explicitly set, which led to some confusion over the default
behavior for the nullable column value when setting a primary_key.

Looking at the docs it's not clear, but the code shows that if
nullable is not specified, then nullable = not primary_key, so
nullable defaults to False when primary_key is True.

This patch adds a simple clarification to the docs so people
don't have to check the code.

Change-Id: I8553339d56fbae11370c7c6af6d8d4723163be1c
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/312

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