DOC: SapphireTest `objFromFixture`

The current docs for `objFromFixture` cause PHPStorm to generate an error because of the mismatched data types.

`$product = $this->objFromFixture('ProductPage', 'StandardSpecs');`

Causes: `Expected The, got string`.
Merge pull request #2884 from TomSpeak/patch-2

DOC: SapphireTest `objFromFixture`
Added 3.1.3-rc2 changelog