Clarify return value of AccessTokenInterface:setAccessToken

[guillerodriguez]

Different implementations of AccessTokenInterface:setAccessToken seem to follow different guidelines re. the return value. See for example the following:

• The AccessTokenInterface spec does not actually specify a return type.
• The Pdo:setAccessToken method is specified to return a bool value.
• The JwtAccessToken:setAccessToken implementation returns a value if the token should be stored, but does not return anything if no storage is configured (which is equivalent to returning null).

Can you clarify what should be the return type of setAccessToken and where/how is it used?