Add comments to differentiate two cache paths (flutter#9721)

shell/common/switches.h

@@ -49,7 +49,11 @@ DEF_SWITCH(IsolateSnapshotInstructions,
            "isolate-snapshot-instr",
            "The isolate instructions snapshot that will be memory mapped as "
            "read and executable. SnapshotAssetPath must be present.")
-DEF_SWITCH(CacheDirPath, "cache-dir-path", "Path to the cache directory.")
+DEF_SWITCH(CacheDirPath,
+           "cache-dir-path",
+           "Path to the cache directory. "
+           "This is different from the persistent_cache_path in embedder.h, "
+           "which is used for Skia shader cache.")
 DEF_SWITCH(ICUDataFilePath, "icu-data-file-path", "Path to the ICU data file.")
 DEF_SWITCH(ICUSymbolPrefix,
            "icu-symbol-prefix",

shell/platform/embedder/embedder.h

@@ -653,6 +653,9 @@ typedef struct {
   // Path to a directory used to store data that is cached across runs of a
   // Flutter application (such as compiled shader programs used by Skia).
   // This is optional.  The string must be NULL terminated.
+  //
+  // This is different from the cache-path-dir argument defined in switches.h,
+  // which is used in |flutter::Settings| as |temp_directory_path|.
   const char* persistent_cache_path;
 
   // If true, we'll only read the existing cache, but not write new ones.