Clarify some of the workings of the YearMonthDayCalculator cache, fixing one minor comment typo, and switching the order of an expression, but mostly just adding additional prose.
Clarify some of the workings of the YearMonthDayCalculator cache,
fixing one minor comment typo, and switching the order of an expression, but
mostly just adding additional prose.

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