Update WindowsBuild.md

Add a note about reducing parallel link operations for memory exhaustion when building.  Thanks to @BradLarson for pointing out that this may be something that may not be obvious to all users.

Author: compnerd

docs/WindowsBuild.md

@@ -135,6 +135,8 @@ cmake -B "S:\b\1" ^
 ninja -C S:\b\1
 ```
 
+> **NOTE:** Linking with debug information (`-D LLVM_ENABLE_PDB=YES`) is very memory intensive.  When building with parallel jobs, it is possible to consume upwards of 32 GiB of RAM.  You can append `-D LLVM_PARALLEL_LINK_JOBS=N -D DLLVM_PARALLEL_LINK_JOBS=N` to reduce the number of parallel link operations to `N` which should help reduce the memory pressure.  You may need to set this to a low number (e.g. 1) if you see build failures due to memory exhaustion.
+
 ## Running Swift tests on Windows
 
 ```cmd