add doc comment on "P"/"PS" struct

src/binary_formats.ml

@@ -1,10 +1,23 @@
 (* https://docs.taler.net/core/api-common.html#binary-formats
 
    - numeric values are in network byte order (big endian) *)
-(* TODO: ?
-   - some struct have a 'P' suffix, but are not defined in doc
-     we assume they are = to the un-suffixed ones
-   - some purpose (`TALER_SIGNATURE_XXX`) are missing or outdated(?) *)
+
+(* structs that are ‘packed’ and do not contain pointers and are
+   thus suitable for hashing or similar operations are distinguished
+   by adding a “P” at the end of the name. (NEW) Note that this convention
+   does not hold for the GNUnet-structs (yet).
+
+   structs that are used with a purpose for signatures,
+   additionally get an “S” at the end of the name.
+
+   (from https://docs.taler.net/taler-developer-manual.html) *)
+
+(* TODO
+   - check that our struct are well "packed"
+   - test them
+   - some purpose (`TALER_SIGNATURE_XXX`) are missing
+     exchange and gana master branch are not in sync
+     and we should use a specific git tag instead *)
 
 open Include