Document that Solution variables are only available sometimes (#242)

You can only use Solution variables if you pass a .sln file to msbuild.

src/Docusaurus/docs/tools/msbuild.md

@@ -100,6 +100,11 @@ The WiX MSBuild targets create a number of preprocessor variables for each refer
 | _ProjectName_.TargetName | $(MyProject.TargetName) | MyProject |
 | _ProjectName_.TargetPath | $(MyProject.TargetPath) | C:\source\repos\ConsoleApp42\bin\Release\MyProject.exe |
 | _ProjectName_.Culture.TargetPath | $(MyProject.en-US.TargetPath) | C:\source\repos\ConsoleApp42\bin\Release\en-US\MyProject.msi |
+
+The above variables are always available. The below variables are only available if you pass a `.sln` file to msbuild, but not if you build an individual project in the solution.
+
+| Variable | Example | Example value |
+| -------- | ------- | ------------- |
 | SolutionDir | $(SolutionDir) | C:\source\repos\MySolution\ |
 | SolutionExt | $(SolutionExt) | .sln |
 | SolutionFileName | $(SolutionFileName) | MySolution.sln |