object.di: Update comment on `clear` deprecation

per discussion at https://github.com/D-Programming-Language/druntime/pull/999
Please change the comment to

    // Explicitly undocumented. It will be removed in March 2015.

and put the same comment on it in object_.d. If the comment does not follow that exact format, the risk of it being missed is much higher - it's what I grep for, and none of the rest of the information is needed.