Remove extra titles from doc pages

Author: Photonios

docs/hstore.md

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-# HStoreField
 `psqlextra.fields.HStoreField` is based on Django's [HStoreField](https://docs.djangoproject.com/en/1.10/ref/contrib/postgres/fields/#hstorefield) and therefore supports everything Django does natively, plus more.
 
 ## Unique constraint

docs/manager.md

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-# Upsert
 An "upsert" is an operation where a piece of data is inserted/created if it doesn't exist yet and updated (overwritten) when it already exists. Django has long provided this functionality through [`update_or_create`](https://docs.djangoproject.com/en/1.10/ref/models/querysets/#update-or-create). It does this by first checking whether the record exists and creating it not.
 
 The major problem with this approach is possibility of race conditions. In between the `SELECT` and `INSERT`, another process could perform the `INSERT`. The last `INSERT` would most likely fail because it would be duplicating a `UNIQUE` constraint.

docs/signals.md

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-# Signals
 Django has long supported "signals", a feature that can be really useful. It allows you to get notified in a callback when a certain event occurs. One of the most common use cases is connecting to model signals. These signals get triggered when a model gets saved, or deleted.
 
 Django's built-in signals have one major flaw: