Update README.md

Author: RobertFlatt

README.md

@@ -208,17 +208,35 @@ Always do this. It is sometimes critically important to disconnect the camera wh
     def capture_screenshot(self, kwargs):
     def capture_video(self, kwargs):      # Android only
     def stop_capture_video(self):         # Android only
-```    
-   Optional arguments:
+```
+
+Captures are saved to '<location>/<date>/<time>.jpg' or '.mp4'.
+
+The default values are as follows. On a desktop <location> is '.', on Android <location> is 'DCIM/<appname>'. The value of <date> is the current date, the format is 'YYYY_MM_DD'. The value of <time> is the current time, the format is 'hh_mm_ss_xx' (xx is 1/100 sec).
+
+The [filepath_callback](https://github.com/Android-for-Python/Camera4Kivy#filepath_callback) reports the actual path for a particular capture.
+
+Be aware that on Android >= 10 shared storage files are saved in a database, called MediaStore, and not in a file system. The characteristics of Android storage are outside the scope of this document. 
+
+The values of <location>, <date>, <time> can be modified with optional keyword arguments:
 
 ##### location :
-A directory that must exist. Default '.' except on Android where can be 'shared' (default) or 'private'.
+
+The value replaces <location>.
+
+On a desktop the value is directory that must exist. 
+
+On Android the value can only be 'shared' or 'private', other values are ignored. The value 'shared' specifies Android shared storage 'DCIM/<appname>' (this is also the default). The value 'private' specifies app local storage 'app_storage_path()/DCIM'. If you want a different location use 'private' and move the resulting file based on the path provided by filepath_callback.
 
 ##### subdir
-A single directory that will be created. Default value is the date.
+
+The value replaces <date>, the subdirectory will be created. 
 
 ##### name
-The file name, default value is the time and '.jpg' or '.mp4'
+
+The value replaces <time>, the '.jpg' or '.mp4' extensions will be added automatically.
+
+Note that it is a characteristic of Android >= 10 shared storage that a second capture with a same subdir and name values as the first, will not overwrite the first. It will create a second file named '<subdir>/<name> (1).jpg', the name is created by Android MediaStore. The MediaStore may crash if it creates too many (31 ?) such names.
 
 #### Select Camera