[Doxygen] Enable autobrief, as claimed by the manual.

Clarify manual to state that first *sentence* will be used as the
brief documentation.

Thanks to Budiarto Herman <budiarto.herman@magister.fi> for pointing
out the required configuration.
Add a custom typehandler for std::ios::openmode, fixes #1815
merge heads
Preserve packet tags upon IPv4 fragmentation
Update RELEASE_NOTES for release
Added tag ns-3.19 for changeset 322102df792e
start new version RELEASE_NOTES