Fix C# client warnings

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Fix C# client warnings. Here are some examples:

src/Org.OpenAPITools/Api/AnotherFakeApi.cs(84,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IAnotherFakeApi'
src/Org.OpenAPITools/Api/FakeApi.cs(654,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IFakeApi'
src/Org.OpenAPITools/Api/FakeClassnameTags123Api.cs(84,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IFakeClassnameTags123Api'
src/Org.OpenAPITools/Api/PetApi.cs(448,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IPetApi'
src/Org.OpenAPITools/Api/StoreApi.cs(206,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IStoreApi'
src/Org.OpenAPITools/Api/UserApi.cs(382,22): warning CS1591: Missing XML comment for publicly visible type or member `Org.OpenAPITools.Api.IUserApi'

cc @mandrean (2017/08) @jimschubert (2017/09)