Explicit example of partial indexes

[PowerKiKi]

This provide an explicit example of partial indexes annotations. Specifically including all parentheses as returned by PostgreSql.

This is needed following doctrine/dbal#716

[doctrinebot]

Hello,

thank you for creating this pull request. I have automatically opened an issue
on our Jira Bug Tracker for you. See the issue link:

http://www.doctrine-project.org/jira/browse/DDC-3465

We use Jira to track the state of pull requests and the versions they got
included in.

[Ocramius]

@PowerKiKi can you split the examples about partial indexes btw? I think they are quite misleading if integrated with "the basics" :-\

[PowerKiKi]

@Ocramius updated the PR as you suggested. Thanks for the feedback.

[deeky666]

Maybe we should add a notice about possible issues using the schema tool to update a schema if the where string does not exactly match the one returned by the database? Not sure... /cc @Ocramius
Otherwise we might get tickets about the schema tool being broken even though it rather is a limitation.

[PowerKiKi]

I'll wait for the confirmation from @Ocramius, but in that case I would like to avoid duplicating that notice, and centralize everything about partial indexes in a single place. However I don't know where that would best fit... do you have an idea ? could it be at the bottom of that page ? or another page ?

[Ocramius]

Maybe we should add a notice about possible issues using the schema tool to update a schema if the where string does not exactly match the one returned by the database?

Don't think so

[Ocramius]

Merged, thanks!