expand comment to explain user overrides

Author: kastiglione

utils/build-script-impl

@@ -584,10 +584,15 @@ function set_build_options_for_host() {
 
             cmake_os_sysroot="$(xcrun --sdk ${platform} --show-sdk-path)"
 
-            # Give the SDK sysroot higher priority in calls to CMake `find_*`
-            # functions, over paths that may come from `pkg-config`. This fixes
-            # compile failures caused when `pkg-config` returns CommandLineTools
-            # paths instead of Xcode paths.
+            # Customize the CMake `find_*` search paths to search the SDK
+            # sysroot before searching `pkg-config` paths. This prevents compile
+            # failures caused when `pkg-config` returns CommandLineTools paths
+            # instead of Xcode paths.
+            #
+            # These search paths can be user customized either by setting the
+            # `CMAKE_PREFIX_PATH` environment variable, or by setting the same
+            # CMake variable via the command line (`-DCMAKE_PREFIX_PATH`).
+            # See https://cmake.org/cmake/help/latest/command/find_path.html
             export CMAKE_PREFIX_PATH="${CMAKE_PREFIX_PATH}:${cmake_os_sysroot}/usr"
 
             cmark_cmake_options=(