Document API joining

Created by: seancolsen

API joining was introduced in 4.7 with CRM-17795. I think it deserves its own page in the Developer Guide, along with API Chaining, etc

There is already some documentation in this PR comment.

More content from the help box in the API explorer:

Join this api call to return values from or filter on a related entity. The new fields will be selectable from the "Params" and "Fields to return" lists. Example: If the api entity is Email, you can also fetch the display name of the contact this email belongs to. Disambiguation: Joins are different from api chaining in that joins execute as a single SELECT query, and only work for api "get" operations.

I think it'd be nice to create a page in the Dev Guide and then simplify the above help text to link to that page