Document field extensions support

Update issue 17
Status: FixedInGit

docs/reference.rst

@@ -304,6 +304,41 @@ Protocol Buffers wire types. These are used with `pb_encode_tag`_. ::
         PB_WT_32BIT  = 5
     } pb_wire_type_t;
 
+pb_extension_type_t
+-------------------
+Defines the handler functions and auxiliary data for a field that extends
+another message. Usually autogenerated by *nanopb_generator.py*::
+
+    typedef struct {
+        bool (*decode)(pb_istream_t *stream, pb_extension_t *extension,
+                   uint32_t tag, pb_wire_type_t wire_type);
+        bool (*encode)(pb_ostream_t *stream, const pb_extension_t *extension);
+        const void *arg;
+    } pb_extension_type_t;
+
+In the normal case, the function pointers are *NULL* and the decoder and
+encoder use their internal implementations. The internal implementations
+assume that *arg* points to a *pb_field_t* that describes the field in question.
+
+To implement custom processing of unknown fields, you can provide pointers
+to your own functions. Their functionality is mostly the same as for normal
+callback fields, except that they get called for any unknown field when decoding.
+
+pb_extension_t
+--------------
+Ties together the extension field type and the storage for the field value::
+
+    typedef struct {
+        const pb_extension_type_t *type;
+        void *dest;
+        pb_extension_t *next;
+    } pb_extension_t;
+
+:type:      Pointer to the structure that defines the callback functions.
+:dest:      Pointer to the variable that stores the field value
+            (as used by the default extension callback functions.)
+:next:      Pointer to the next extension handler, or *NULL*.
+
 PB_GET_ERROR
 ------------
 Get the current error message from a stream, or a placeholder string if