core(persistence): fix writeRaw() / readRaw() struct support

- writeRaw(): support structs
- readRaw(): 'len' is buffer limit in bytes (documentation is fixed)

modules/core/include/opencv2/core/persistence.hpp

@@ -597,8 +597,8 @@ public:
     Usually it is more convenient to use operator `>>` instead of this method.
     @param fmt Specification of each array element. See @ref format_spec "format specification"
     @param vec Pointer to the destination array.
-    @param len Number of elements to read. If it is greater than number of remaining elements then all
-    of them will be read.
+    @param len Number of bytes to read (buffer size limit). If it is greater than number of
+               remaining elements then all of them will be read.
      */
     void readRaw( const String& fmt, uchar* vec, size_t len ) const;
 
@@ -668,11 +668,12 @@ public:
     Usually it is more convenient to use operator `>>` instead of this method.
     @param fmt Specification of each array element. See @ref format_spec "format specification"
     @param vec Pointer to the destination array.
-    @param maxCount Number of elements to read. If it is greater than number of remaining elements then
-    all of them will be read.
+    @param len Number of bytes to read (buffer size limit). If it is greater than number of
+               remaining elements then all of them will be read.
+
      */
     FileNodeIterator& readRaw( const String& fmt, uchar* vec,
-                               size_t maxCount=(size_t)INT_MAX );
+                               size_t len=(size_t)INT_MAX );
 
     struct SeqReader
     {