Fix ConvertCSharpDocs multi-line regression

[lahma]

Reverted back to old logic, added unit test, changed summary format to span multiple lines so multi-line ones don't look weird.

fixes #1462
fixes #3793

[RicoSuter]

It's a dangerous fix as it requires a template change (some might have custom templates).

[lahma]

Old templates should work as-is. Now the built-in ones just add new line before and after the actual content:

/// <summary>OLD_INSERTED_DATA</summary>

/// <summary>
/// OLD_INSERTED_DATA
/// </summary>

The c# part is the sample, it just appends the same data to different valid doc comment section.

[RicoSuter]

Need to fix those in NSwag as well, right?

[lahma]

These should work without changes, but syncing the logic of having separate line might make sense.