logd: document ro.logd.size

Bad comment advise in LogBuffer.cpp results in partners failing to
considering using ro.logd.size to set the platform buffer size
default.

NB: It is not good practice to increase the log buffer size to deal
with logspam, as increases will result in logd scale issues getting
closer to hitting the background cgroup cpu cap. Once we hit that
cap, logd spirals, pruning old entries slower than the incoming log
entries. logd.writer will take 100% cpu.

Change-Id: If4a7a74f300d078eeaed0ffd3eb3fd77d1f9fe90