lib: fix the todo comment placement on NOTMUCH_STATUS_XAPIAN_EXCEPTION

The todo comment got separated from the status it's related to at
commit 3f32fd8a1c06 ("Add missing comment for
NOTMUCH_STATUS_READONLY_DATABASE."). Later, commit b65ca8e0ba5e ("lib:
modify notmuch.h for automatic document generation") moved it, but to
the wrong place. Fix the location.