Bug 15493 – GC.BlkAttr.APPENDABLE refers to a broken link

http://dlang.org/phobos/core_memory.html#.GC.BlkAttr.APPENDABLE It says "Use capacity to retrieve actual useable capacity. " where capacity is a link that leads nowhere. I see no function called capacity anywhere in core.memory that it might be referring to. There's a second reference to this link under `extend`. I *think* it is talking about slice.capacity, which is referenced here http://dlang.org/spec/arrays.html#dynamic-arrays but not formally documented anywhere that we can actually link to.

Should be a link to https://dlang.org/phobos/object.html#.capacity.

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