Document sandbox on getRealFileSystem() and createPhysicalFileSystem()

Author: jansvoboda11

llvm/include/llvm/Support/VirtualFileSystem.h

@@ -372,12 +372,14 @@ class LLVM_ABI FileSystem : public llvm::ThreadSafeRefCountedBase<FileSystem>,
 /// the operating system.
 /// The working directory is linked to the process's working directory.
 /// (This is usually thread-hostile).
+/// This may only be called outside the IO sandbox.
 LLVM_ABI IntrusiveRefCntPtr<FileSystem> getRealFileSystem();
 
 /// Create an \p vfs::FileSystem for the 'real' file system, as seen by
 /// the operating system.
 /// It has its own working directory, independent of (but initially equal to)
 /// that of the process.
+/// This may only be called outside the IO sandbox.
 LLVM_ABI std::unique_ptr<FileSystem> createPhysicalFileSystem();
 
 /// A file system that allows overlaying one \p AbstractFileSystem on top