doc: Improve the monotonic time module and function documentation

[teor2345]

Explain what "monotonic" actually means, and document some results that
have surprised people.

Fixes bug 29640; bugfix on 0.2.9.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 4064

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 61.753%

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Files with Coverage Reduction	New Missed Lines	%
src/core/or/circuitpadding.c	10	93.71%

Totals
Change from base Build 4042:	0.02%
Covered Lines:	45333
Relevant Lines:	73410

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💛 - Coveralls