Cleaned up documentation

Cleaned up documentation, tweaked some of the grammar, changed class#method() references to class::method(), added warning about performance impact of lazy loading
Updated method documentation to use # for instance methods instead of ::
Update getting-started.rst

Replaced ``class::$field`` with ``class#field`` to match Doctrine style

Cleaned up three paragraphs mentioned in https://github.com/doctrine/doctrine2/pull/734
Merge pull request #734 from caponica/patch-1

Cleaned up documentation