Add docs for the Unicode version implemented by each segmenter

[sffc]

The docs of each segmenter type (WordSegmenter, LineSegmenter, SentenceSegmenter, GraphemeClusterSegmenter) should have a sentence such as

This segmenter conforms to Unicode 17.0

And we should update that doc string whenever we update the underlying data.

CC @aethanyc @makotokato

[aethanyc]

We also need to update the comment at the beginning of the segmenter toml rule files to reflect the reality.

For example, these lines are probably out of date after #7128 updates UAX#29 segmenter to Unicode 17.0.

icu4x/provider/source/data/segmenter/grapheme.toml

Lines 5 to 6 in c46d47f

	# These grapheme boundary rules are based on UAX #29, Unicode Version 16.0.0.
	# https://www.unicode.org/reports/tr29/tr29-45.html

icu4x/provider/source/data/segmenter/sentence.toml

Lines 5 to 6 in c46d47f

	# These sentence boundary rules are based on UAX #29, Unicode Version 16.0.0.
	# https://www.unicode.org/reports/tr29/tr29-45.html

icu4x/provider/source/data/segmenter/word.toml

Lines 5 to 6 in c46d47f

	# These word boundary rules are based on UAX #29, Unicode Version 16.0.0.
	# https://www.unicode.org/reports/tr29/tr29-45.html