bundle2: clarify stream parameter design in the documentation

Stream level parameter have very restricted use case. Clarify why we chosen a
textual format and point that applicative data goes in applicative parts.

mercurial/bundle2.py

@@ -48,7 +48,14 b' Binary format is as follow'
 
   Special character in param name are not supported yet.
 
+  Stream parameters use a simple textual format for two main reasons:
 
+  - Stream level parameters should remains simple and we want to discourage any
+    crazy usage.
+  - Textual data allow easy human inspection of a the bundle2 header in case of
+    troubles.
+
+  Any Applicative level options MUST go into a bundle2 part instead.
 
 
 Payload part