There are four possible states for the WT_PAGE.dsk memory: the WT_PAGE
disk-image is mapped into memory (unmap on page discard), is in allocated
memory (free on page discard), is in memory belonging to a caller (ignore on
page discard), or isn't set at all (ignore on page discard).

We've been trying to figure out which based on the state of WT_PAGE.dsk and
a single flag, and that's not enough.  Use two explicit flags, one for
mapped memory and one for allocated memory, and neither flag is set for the
latter two states.
Rework the documentation & examples to better reflect "overwrite" is
configured on/true by default, and should only be set if you want
non-standard behavior.