Document MSBuildDefiningProject* item metadata

MSBuild now (as of version 14 or maybe a 12 update?) supports well-known metadata of the form [`MSBuildDefiningProject*`](https://github.com/Microsoft/msbuild/blob/0342905d1bbebff671687bd7163cf5ac31d26de5/src/Shared/Modifiers.cs#L54-L57). That should be documented at https://docs.microsoft.com/en-us/visualstudio/msbuild/msbuild-well-known-item-metadata.

Also probably worth updating the [comment on `GetItemSpecModifier`](https://github.com/Microsoft/msbuild/blob/0342905d1bbebff671687bd7163cf5ac31d26de5/src/Shared/Modifiers.cs#L353-L364) that lists the supported ones.

Internal changes that might be useful:
http://vstfdevdiv:8080/DevDiv2/DevDiv/_versionControl/changeset/1065156?_a=compare&path=%24%2FDevDiv%2Fd12u%2FPhoneUDev%2Fxmake%2FShared%2FModifiers.cs