[refactor](FileCache) set FE session variable enable_file_cache=false as default (#19327)

Users should set `enable_file_cache=true` in FE session variables and BE configuration to enable file cache.

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -802,11 +802,18 @@ public class SessionVariable implements Serializable, Writable {
     public boolean groupByAndHavingUseAliasFirst = false;
 
     // Whether enable block file cache. Only take effect when BE config item enable_file_cache is true.
-    @VariableMgr.VarAttr(name = ENABLE_FILE_CACHE, needForward = true)
-    public boolean enableFileCache = true;
+    @VariableMgr.VarAttr(name = ENABLE_FILE_CACHE, needForward = true, description = {
+            "是否启用file cache。该变量只有在be.conf中enable_file_cache=true时才有效，"
+                    + "如果be.conf中enable_file_cache=false，该BE节点的file cache处于禁用状态。",
+            "Set wether to use file cache. This variable takes effect only if the BE config enable_file_cache=true. "
+                    + "The cache is not used when BE config enable_file_cache=false."})
+    public boolean enableFileCache = false;
 
     // Specify base path for file cache, or chose a random path.
-    @VariableMgr.VarAttr(name = FILE_CACHE_BASE_PATH, needForward = true)
+    @VariableMgr.VarAttr(name = FILE_CACHE_BASE_PATH, needForward = true, description = {
+            "指定block file cache在BE上的存储路径，默认 'random'，随机选择BE配置的存储路径。",
+            "Specify the storage path of the block file cache on BE, default 'random', "
+                    + "and randomly select the storage path configured by BE."})
     public String fileCacheBasePath = "random";
 
     // Whether drop table when create table as select insert data appear error.