[C#] Convert Javadoc in C# code to proper triple-slash documentation

[RenderMichael]

I like this library and would like to give back: This PR converts the Javadoc in the C# source code to C# XML comments. With these changes, each class's documentation would be made usable by IDE's.

Notably, this PR does not affect functionality, compat, etc. in any way, so there is no risk accepting this PR. Aside from the documentation convertion, my IDE auto-formatted the opening curly brace to start on the next line, as is the norm in C#.

Edit:
This PR is easier to review with whitespace ignored: https://github.com/google/diff-match-patch/pull/128/files?w=1

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