Added detailed BC promise text
Updated BC promise to be valid as of Symfony 2.3
Rearranged rules to be more easily understandable
Added information about type compatibility
Added tables with safe operations
Fixed: No parameters must be added (ever) to API class methods
Changed: The last parameters of a method may be removed
Rearranged safe operations to make more sense
Removed most of the "cannot" statements which are repeated in the tables now
Improved wording
Added: Some breaking changes to unsafe operations are documented in the UPGRADE file
Rearranged page to have different sections for different user bases
Language fixes
Fixed headings
Language improvements
Fixed safety statements
Added that adding custom properties is not safe
Improved wording
Added note about requesting `@api` tags
Added information about internal classes and interfaces