update docs (#92)

manual/custom_rule.md

@@ -564,7 +564,7 @@ target("test")
 #### utils.symbols.export_all
 
 Provided in v2.5.2 and above, we can use it to automatically export all dynamic library symbols. Currently, only the symbol export of windows dll target programs is supported, even if there is no export interface through `__declspec(dllexport)` in the code.
-xmake will also automatically export all c interface symbols (there are too many c++ class library symbols, so I haven't exported them yet).
+xmake will also automatically export all c/c++ interface symbols.
 
 ```lua
 add_rules("mode.release", "mode.debug")
@@ -580,6 +580,11 @@ target("test")
      add_files("src/main.c")
 ```
 
+c++
+```lua
+add_rules("utils.symbols.export_all", {export_classes = true})
+```
+
 Related issue [#1123](https://github.com/xmake-io/xmake/issues/1123)
 
 #### utils.symbols.export_list

zh-cn/manual/custom_rule.md

@@ -566,7 +566,7 @@ target("test")
 #### utils.symbols.export_all
 
 v2.5.2 以上版本提供，我们可以用它自动导出所有的动态库符号，目前仅支持 windows dll 目标程序的符号导出，即使没有在代码中通过 `__declspec(dllexport)` 导出接口，
-xmake 也会自动导出所有 c 接口符号（c++ 类库符号太多，暂时没有导出它）。
+xmake 也会自动导出所有 c/c++ 接口符号。
 
 ```lua
 add_rules("mode.release", "mode.debug")
@@ -582,6 +582,11 @@ target("test")
     add_files("src/main.c")
 ```
 
+c++
+```lua
+add_rules("utils.symbols.export_all", {export_classes = true})
+```
+
 相关 issue [#1123](https://github.com/xmake-io/xmake/issues/1123)
 
 #### utils.symbols.export_list