Enhance documentation of API

[stevepiercy]

Fixes #631.

Depends on #636 for change log consistency.

I also fixed a docstring that returned this error:

/Users/stevepiercy/projects/icalendar/src/icalendar/prop.py:docstring of icalendar.prop:4: CRITICAL: Missing matching underline for section title overline.

###########################################################################
# This module defines these property value data types and property parameters

Sphinx treated that line as a page title, and a blank line resolved the issue.

I'm not sure whether you want that line to be a comment or displayed, though.

[coveralls]

Pull Request Test Coverage Report for Build 9606426632

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.228%

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Totals
Change from base Build 9598258374:	0.0%
Covered Lines:	3087
Relevant Lines:	3175

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9606439009

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 97.228%

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Files with Coverage Reduction	New Missed Lines	%
src/icalendar/timezone/zoneinfo.py	2	94.67%
src/icalendar/tests/test_cli_tool.py	2	88.0%

Totals
Change from base Build 9598258374:	0.0%
Covered Lines:	3087
Relevant Lines:	3175

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💛 - Coveralls

[niccokunzmann]

Thanks!