Fix LLVM_EXTERNALIZE_DEBUGINFO docs, and add new options from patchset

[directhex]

Summary

This fixes the "Mac only" comment in the documentation, and adds the new options added in my related patches

llvm-project/llvm/docs/CMake.rst

   -DLLVM_EXTERNAL_BAR_SOURCE_DIR=/src/bar``.
 
 **LLVM_EXTERNALIZE_DEBUGINFO**:BOOL
-  Generate dSYM files and strip executables and libraries (Darwin Only).
+  Generate dSYM files and strip executables and libraries. On Windows, debug
+  info (pdb files) is always external, and always produced on Debug or
+  RelWithDebInfo builds, or Release builds with LLVM_ENABLE_PDB.
   Defaults to OFF.
 
+**LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION**:STRING
+  Specify the file extension used by LLVM_EXTERNALIZE_DEBUGINFO. Defaults to
+  .debug on Linux, and .dSYM on macOS unless LLVM_EXTERNALIZE_DEBUGINFO_FLATTEN
+  is enabled, which defaults to .dwarf.
+
+**LLVM_EXTERNALIZE_DEBUGINFO_FLATTEN**:BOOL
+  Tell dsymutil to emit a simple "flat" debug file, rather than a full debug
+  directory structure. macOS only.
+  Defaults to OFF.
+
+**LLVM_EXTERNALIZE_DEBUGINFO_INSTALL**:BOOL
+  Install debug symbols alongside executables, during call to cmake install.
+  Also enables installing .pdb files on Windows.
+  Defaults to OFF.
+
+**LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR**:STRING
+  Override the directory where debug symbols are produced. Defaults to no value
+  (place debug symbols next to their corresponding executables)
+
 **LLVM_FORCE_USE_OLD_TOOLCHAIN**:BOOL
   If enabled, the compiler and standard library versions won't be checked. LLVM
   may not compile at all, or might fail at runtime due to known bugs in these