qapi: Document optional arguments' backwards compatibility

 { '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