[runtimes] Improve the documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS (#112733)

This clarifies how that option is meant to be used to avoid confusion.
As a drive-by, also fix an incorrect usage in the recently-added GPU
caches.

NOKEYCHECK=True
GitOrigin-RevId: e67442486d5efd48235f62b438557bc95193fc48

Author: ldionne

CMakeLists.txt

@@ -162,8 +162,7 @@ set(LIBUNWIND_C_FLAGS "")
 set(LIBUNWIND_CXX_FLAGS "")
 set(LIBUNWIND_COMPILE_FLAGS "")
 set(LIBUNWIND_LINK_FLAGS "")
-set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING
-    "Additional Compile only flags which can be provided in cache")
+set(LIBUNWIND_ADDITIONAL_COMPILE_FLAGS "" CACHE STRING "See documentation for LIBCXX_ADDITIONAL_COMPILE_FLAGS")
 set(LIBUNWIND_ADDITIONAL_LIBRARIES "" CACHE STRING
     "Additional libraries libunwind is linked to which can be provided in cache")