Reword the comment to better describe what I found while researching the
problem that led to this temporary workaround (and also so I can properly
cite the PR in the commit this time).

In general this is intended to be a temporary workaround until we can figure
out why including any ram from the last megabyte of the physical address
space leads to a NULL pointer deref. Debugging that is made trickier by the
fact that I couldn't even get a backtrace in ddb.

PR: 201614