Fix deprecation comment on clear.

It doesn't follow the format that those comments normally take, and the
date isn't good, because while it was committed in March of 2014, it
wasn't actually get released until mid-August with 2.066, and it really
needs to be deprecated for _at least_ 6 months, if not longer, before
we remove it.

I'm not sure that 6 - 7 months is really enough since, since `clear` is in TDPL, but the commit where it was originally deprecated had it at 6 months, and `clear` isn't documented anywhere outside of TDPL.

Related: https://github.com/D-Programming-Language/druntime/pull/999