Update docs to make it clear that fast=false zeros the initialized matrix.
Update docs to make it clear that fast=false zeros the initialized matrix.

roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,dschwen/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,roystgnr/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh