Add note about varying use of BEF/AFT

[tychonievich]

I'm not sure this is the right note, but hope it will be a useful starting point for conversation leading to the right note.

Closes #773

[tychonievich]

Suggested change

	Those definitions suggest that `AFT 1850` would mean "1 JAN 1851 or later,"
	while under the 7.0 definition `AFT 1850` means "1 JAN 1850 or later."
	This change in definitions was made to align with common (but not universal) practice.
	Given a source dated 1850 that asserts something had happened in the past
	(which could mean earlier that same year or in an earlier year),
	some users encode that as `BEF 1850` and others as `BEF 1851`.
	Both user entries are consistent with the source under the 7.0 definition,
	while only the second is consistent with the source under the 5.x definition.
	These definitions left ambiguous whether they meant the beginning or end of the date in question:
	did `BEF 1850` mean "before the beginning of 1850: 31 DEC 1849 or before"
	or did it mean "before the end of 1850: 31 DEC 1850 or before"?
	Some users found the inclusive definition more natural,
	and would encode a source dated 1850 asserting something had happened in the past
	as `BEF 1850`;
	others preferred the more natural linguistic form of the exclusive definition,
	and entered the same source as `BEF 1851`
	because they didn't know if it happened earlier in 1850 or an earlier year.
	Choosing one over the other also seemed to vary by region,
	with some dialects and translations interpreting "before" and "after" differently than others.
	The updated definition text in 7.0 is intended to be unambiguously inclusive:
	`BEF 1850` could mean sometime within 1850, or before that date began.
	The inclusive definition was chosen because it does not contradict the exclusive definition:
	interpreting exclusive intent as inclusive data loses precision,
	while interpreting inclusive intend as exclusive asserts falsehoods.

This alternative wording points out that there was an actual ambiguity, not just a sloppiness of use, in the 5.x wording, but also doesn't suggest that disagreements still exist. I can't decide if I like this text better or not.