Normative: Add wording for buddhist calendar complexities for pre-1941 dates

[ben-allen]

As the result of the change of the new year in the Thai Buddhist calendar from April to January that occurred on 1 January 1941, calculating dates from before that year must take into account that the preceding year only had nine months.

This is currently a draft PR because I am not entirely satisfied with the wording -- I'm genuinely not sure what level of formality is appropriate in this context.

See #63

[Manishearth]

Do we wish to do this? The calendar is supposed to be proleptic.

Probably not a big deal, but no current implementor follows this tweak.

[ben-allen]

Do we wish to do this? The calendar is supposed to be proleptic.

I think the answer is in fact no -- which when you get down to it is the key reason why I've left this as a draft PR.

However, would it be useful to have an explicit mention of the nine month year that occurred in 1940 in the description of the calendar? See the paragraph on the Buddhist calendar description in this comment from @sffc

[ptomato]

My guess is that we want wording like this, only it should say the opposite, i.e. years begin in January extending indefinitely into the past.

[sffc]

I never did research on the pre-1941 Buddhist question. Is it still Gregorian-based, just with the new year shifted, or is it a whole other type of calendar?

[ben-allen]

It's the Gregorian calendar, just with the new year shifted -- they did the shift by having one nine-month year.

[ben-allen]

Changed the wording to do, as @ptomato says, the opposite of that.