Fix AddGeneratedFile target's Outputs file path

[ratijas]

Setting correct Output should reduce extra work on recompilation.

There is a duplication of MyPluginInfo.cs file path, and it doesn't seem to be possible to set it in a PropertyGroup outside of Target.

Also add brief comments on why there are seemingly two similar items about writing to file.

[ratijas]

For the context, documentation on topic:

https://learn.microsoft.com/en-us/visualstudio/msbuild/target-element-msbuild?view=visualstudio

Outputs - Optional attribute.

The files that form outputs into this target. Multiple files are separated by semicolons. The timestamps of the files will be compared with the timestamps of files in Inputs to determine whether the Target is up to date. For more information, see Incremental builds, How to: Build incrementally, and Transforms.