The BC policy table needs to be updated to cover return types

[stof]

Our BC policy table listing everything contributors can and cannot do due to BC currently explains when you can add or remove typehints for arguments.
We need to update this table to explain what we can do for return types as well, given that Symfony 4 will be able to use them (and will use them when possible)

[alcaeus]

There are Change return type lines in the tables at https://symfony.com/doc/current/contributing/code/bc.html#working-on-symfony-code - shouldn't that be sufficient?

[javiereguiluz]

Closing as "not needed to be fixed" because most people on Symfony Slack thought that we don't need to do anything else here. Thanks!

[stof]

We need rules about adding or removing a PHP return type, similar to what we have for typehints. (removing an explicit return type, even without changing the contract in the documented return type, is a BC break in PHP)

[nicolas-grekas]

see #9717

[javiereguiluz]

Closing as fixed by #9717.