Fix for documentation of Covering Indexes

Hi all,

The documentation of covering indexes is incorrect for CREATE and ALTER
TABLE:
- ALTER TABLE's page is missing the call.
- Exclusion constraints can use INCLUDE clauses.

In order to simplify the documentation, please let me suggest the
attached which moves the definition of the INCLUDE clause into the
section index_parameters, which is compatible with what I read from the
parser.

Thanks,
--
Michael