Change method descriptions to be more consistent

All doc comments should follow consistent patterns.

• <param name="self">Instance of extended class.</param>
• <exception cref="NullReferenceException">If this instance is <c>null</c>.</exception>
• <returns>A fluent reference to <c>this</c> <see cref="SomeClass"/> instance.</returns>
• <returns><c>true</c> if some condition; otherwise <c>false</c>.</returns> ^
• CountMatches(): Reports the number of occurrences of in this instance.
• Formatting Methods: Returns a copy of this instance with [...]
• Comparison Methods: Determines if this instance [...]
• Conversion Methods: Returns a copy of this instance with [...]

Some examples in the doc comments could be helpful: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/xmldoc/example

Extension Classes

/// <summary>
/// The <see cref="ClassName"/> <c>static</c> <c>extension</c> <c>class</c> provides
/// extension methods related to <see cref="SubjectClass"/> objects.
/// </summary>

Helper Classes

/// <summary>
/// The <see cref="HelperClassName"/> <c>static</c> <c>helper</c> <c>class</c> provides
/// methods related to <see cref="SubjectClass"/> objects.
/// </summary>