Mention warning in OnPrem TFS in documentation

Author: Pascal Berger

docs/more-info/build-server-setup/tfs-build-vnext.md

@@ -38,7 +38,7 @@ GitVersion passes variables in the form of `GitVersion.*` (Eg: `GitVersion.Major
 See [Variables](/more-info/variables/) for an overview of available variables.
 
 #### Known limitations
-* Due to [current limitations in TFS2015 On-Prem](https://github.com/Microsoft/vso-agent-tasks/issues/380) it's currently not possible to automatically set the build version in TFS2015 On-Prem.
+* Due to [current limitations in TFS2015 On-Prem](https://github.com/Microsoft/vso-agent-tasks/issues/380) it's currently not possible to automatically set the build version in TFS2015 On-Prem. Instead a warning similar to `##[warning]Unable to process logging event:##vso[build.updatebuildnumber 1.0.0-unstable.1` is logged.
 * Due to a know limitation in TFS 2015 On-Prem it's currently not possible to use variables added during build in inputs of subsequent build tasks, since the variables are processed at the beginning of the build. 
 As a workaround environment variables can be used in custom scripts.