Add comment about multi-targeting projects

Author: AArnott

azure-pipelines/build.yml

@@ -6,7 +6,7 @@ parameters:
 # Whether this repo uses OptProf to optimize the built binaries.
 # When enabling this, be sure to update these files:
 # - OptProf.targets: InstallationPath and match TestCase selection with what's in the VS repo.
-# - The project file(s) for the libraries to optimize must import OptProf.targets.
+# - The project file(s) for the libraries to optimize must import OptProf.targets (for multi-targeted projects, only import it for ONE target).
 # - OptProf.yml: Search for LibraryName (or your library's name) and verify that those names are appropriate.
 # - OptProf_part2.yml: Search for LibraryName (or your library's name) and verify that those names are appropriate.
 # and create pipelines for OptProf.yml, OptProf_part2.yml