Documentation cleanups for Offset.
Documentation cleanups for Offset.

Mostly making everything consistent, but it also doesn't make a lot of sense
to highlight methods as "creating a new Offset" when Offset is a value type.

BenJenkinson/nodatime,zaccharles/nodatime,zaccharles/nodatime,malcolmr/nodatime,malcolmr/nodatime,zaccharles/nodatime,jskeet/nodatime,zaccharles/nodatime,zaccharles/nodatime,nodatime/nodatime,malcolmr/nodatime,jskeet/nodatime,zaccharles/nodatime,nodatime/nodatime,malcolmr/nodatime,BenJenkinson/nodatime