Document that comments aren't preserved for round trip

[takluyver]

From discussion on #1052.

[coveralls]

Pull Request Test Coverage Report for Build 7052192921

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 87.706%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	1	11.92%

Totals
Change from base Build 7009122777:	0.0%
Covered Lines:	17992
Relevant Lines:	20514

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💛 - Coveralls

[alamb]

Thank you @takluyver 🙏