Document non-thread-safety of RTS API in non-threaded RTS

The RTS API is currently completely unsafe to use in a threaded program linked against the non-threaded API. For instance, createThread calls into the storage manager, yet we compile-out the storage manager lock in the non-threaded RTS. Currently this fact is quite under-represented in the documentation. Fix this.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information