qapi: Document new 'alternate' meta-type

qapi: Document new 'alternate' meta-type

The next patch will quit special-casing "'union':'Foo',
'discriminator':{}" and instead use "'alternate':'Foo'".

Separating docs from implementation makes it easier to focus
on wording without holding up code.  In particular, making
alternate a separate type makes for a nice type hierarchy:

          /-------- meta-type ------\
         /              |            \
    simple types    alternate     complex types
    |         |                   |           |
 built-in   enum             type(struct)   union
 |       \    /                            /    \
numeric  string                         simple  flat

A later patch will then clean up 'type' vs. 'struct'
confusion.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>