DDSPSMC11_ — Small correctes to the specification

Some small fixes to the spec

Section 7.1, change "sup- ported" to "supported"

Section 7.4.3, C+11 talks about strong typed enums, so change "C+11 enumeration
classes" to "C++11 strongly typed enums"

Section 7.5.1.2, why let the user initialize a null reference by assigning dds::core::null, wouldn't be be easier to say "The trivial constructor for reference types will initialize it to a null reference"

Section 8.2, constructor of RadarTrack doesn't use optional for argument z