3.0 - Fix join docblocks

@markstory How about something like this, a short description and a reference to `join()` for details?

	* Adds a single LEFT JOIN clause to the query.
	*
	* This is a shorthand method for building joins via `join()`.
	*
	* The table name can be passed as a string, or as an array in case
	* it needs to be aliased:
	*
	* {{{
	* // LEFT JOIN authors ON authors.id = posts.author_id
	* $query->leftJoin('authors', 'authors.id = posts.author_id');
	*
	* // LEFT JOIN authors a ON a.id = posts.author_id
	* $query->leftJoin(['a' => 'authors'], 'a.id = posts.author_id');
	* }}}
	*
	* Conditions can be passed as strings, arrays, or expression objects.
	* When using arrays it is possible to combine them with the `$types`
	* parameter in order to define how to convert the values:
	*
	* {{{
	* $query->leftJoin(['a' => 'articles'], [
	*		'a.posted >=' => new DateTime('-3 days'),
	*		'a.published' => true,
	*		'a.author_id = authors.id'
	* ], ['a.posted' => 'datetime', 'a.published' => 'boolean']);
	* }}}
	*
	* See `join()` for further details on conditions and types.

The other shorthand methods could refer to `leftJoin()` as the usage is identical.

On a side note, I just spotted that there seems to be no need to use double backticks for inline code, is there a reason for them being around, or is it just a mistake?