document clearer behavior for lazy attributes

When the attribute is lazy, it will be regenerated the next time it is
accessed. If it is not lazy, it will remain undef.

I ran across this, expecting my default sub to be called again, but my attribute was not lazy. This matches with Moose's behavior though, so this is a doc-only patch. (see gist https://gist.github.com/preaction/566e2f27954f57d56a74)