qapi: Document optional arguments' backwards compatibility

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

docs/qapi-code-gen.txt

@@ -60,10 +60,34 @@ example of a complex type is:
  { 'type': 'MyType',
    'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } }
 
-The use of '*' as a prefix to the name means the member is optional.  Optional
-members should always be added to the end of the dictionary to preserve
-backwards compatibility.
-
+The use of '*' as a prefix to the name means the member is optional.
+
+The default initialization value of an optional argument should not be changed
+between versions of QEMU unless the new default maintains backward
+compatibility to the user-visible behavior of the old default.
+
+With proper documentation, this policy still allows some flexibility; for
+example, documenting that a default of 0 picks an optimal buffer size allows
+one release to declare the optimal size at 512 while another release declares
+the optimal size at 4096 - the user-visible behavior is not the bytes used by
+the buffer, but the fact that the buffer was optimal size.
+
+On input structures (only mentioned in the 'data' side of a command), changing
+from mandatory to optional is safe (older clients will supply the option, and
+newer clients can benefit from the default); changing from optional to
+mandatory is backwards incompatible (older clients may be omitting the option,
+and must continue to work).
+
+On output structures (only mentioned in the 'returns' side of a command),
+changing from mandatory to optional is in general unsafe (older clients may be
+expecting the field, and could crash if it is missing), although it can be done
+if the only way that the optional argument will be omitted is when it is
+triggered by the presence of a new input flag to the command that older clients
+don't know to send.  Changing from optional to mandatory is safe.
+
+A structure that is used in both input and output of various commands
+must consider the backwards compatibility constraints of both directions
+of use.
 
 A complex type definition can specify another complex type as its base.
 In this case, the fields of the base type are included as top-level fields