Updates to handle included documentation

[vweijsters]

• Fixes Update SA1605/SA1607 to handle included documentation #1899 (SA1605/SA1607)
• Fixes Update SA1609 (PropertyDocumentationMustHaveValue) to handle included documentation #1901 (SA1609)
• Fixes Update SA1610 (PropertyDocumentationMustHaveValueText) to handle included documentation #1903 (SA1610)
• Fixes Update SA1617 (VoidReturnValueMustNotBeDocumented) to handle included documentation #1909 (SA1617)
• Fixes Update SA1618 (GenericTypeParametersMustBeDocumented) to handle included documentation #1910 (SA1618)
• Fixes Update SA1619 (GenericTypeParametersMustBeDocumentedPartialClass) to handle included documentation #1911 (SA1619)
• Fixes Update SA1620 (GenericTypeParameterDocumentationMustMatchTypeParameters) to handle included documentation #1912 (SA1620)
• Fixes Update SA1621 (GenericTypeParameterDocumentationMustDeclareParameterName) to handle included documentation #1913 (SA1621)
• Fixes Update SA1622 (GenericTypeParameterDocumentationMustHaveText) to handle included documentation #1914 (SA1622)
• Fixes Update SA1627 (DocumentationTextMustNotBeEmpty) to handle included documentation #1916 (SA1627)
• Fixes Update SA1642 (ConstructorSummaryDocumentationMustBeginWithStandardText) to handle included documentation #1917 (SA1642)
• Fixes Update SA1643 (DestructorSummaryDocumentationMustBeginWithStandardText) to handle included documentation #1918 (SA1643)
• Fixes Update SA1648 (InheritDocMustBeUsedWithInheritingClass) to handle included documentation #1919 (SA1648)
• Fixes Update SA1651 (DoNotUsePlaceholderElements) to handle included documentation #1920 (SA1651)

[vweijsters]

@sharwell @pdelvo I've updated SA1605/SA1607 to support included documentation. Unfortunately this wasn't trivial due to a limitation in Roslyn. I would like some feedback on the chosen solution.

[codecov-io]

Current coverage is 96.86% (diff: 97.00%)

Merging #2187 into master will increase coverage by 0.04%

@@             master      #2187   diff @@
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  Files           587        586      -1   
  Lines         78680      80777   +2097   
  Methods        3441       3485     +44   
  Messages          0          0           
  Branches       3027       3088     +61   
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+ Hits          76174      78242   +2068   
- Misses         1667       1696     +29   
  Partials        839        839           

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[vweijsters]

It is (finally) ready for review