add parallel dump file info to rerun doc page

doc/src/rerun.txt

@@ -107,6 +107,14 @@ However if you specify a series of dump files in an incorrect order
 (with respect to the timesteps they contain), you may skip large
 numbers of snapshots
 
+Note that the dump files specified as part of the {dump} keyword can
+be parallel files, i.e. written as multiple files either per processor
+and/or per snapshot.  If that is the case they will also be read in
+parallel which can make the rerun command operate dramatically faster
+for large systems.  See the doc page for the "read_dump"_read_dump and
+"dump"_dump.html commands which describe how to read and write
+parallel dump files.
+
 The {first}, {last}, {every}, {skip} keywords determine which
 snapshots are read from the dump file(s).  Snapshots are skipped until
 they have a timestamp >= {Nfirst}.  When a snapshot with a timestamp >