qapi: Relax doc string @name: description indentation rules

qapi: Relax doc string @name: description indentation rules

The QAPI schema doc comment language provides special syntax for
command and event arguments, struct and union members, alternate
branches, enumeration values, and features: descriptions starting with
"@name:".

By convention, we format them like this:

    # @name: Lorem ipsum dolor sit amet, consectetur adipiscing elit,
    #        sed do eiusmod tempor incididunt ut labore et dolore
    #        magna aliqua.

Okay for names as short as "name", but we have much longer ones.  Their
description gets squeezed against the right margin, like this:

    # @dirty-sync-missed-zero-copy: Number of times dirty RAM synchronization could
    #                               not avoid copying dirty pages. This is between
    #                               0 and @dirty-sync-count * @multifd-channels.
    #                               (since 7.1)

The description text is effectively just 50 characters wide.  Easy
enough to read, but can be cumbersome to write.

The awkward squeeze against the right margin makes people go beyond it,
which produces two undesirables: arguments about style, and descriptions
that are unnecessarily hard to read, like this one:

    # @postcopy-vcpu-blocktime: list of the postcopy blocktime per vCPU.  This is
    #                           only present when the postcopy-blocktime migration capability
    #                           is enabled. (Since 3.0)

We could instead format it like

    # @postcopy-vcpu-blocktime:
    # list of the postcopy blocktime per vCPU.  This is only present
    # when the postcopy-blocktime migration capability is
    # enabled. (Since 3.0)

or, since the commit before previous, like

    # @postcopy-vcpu-blocktime:
    # 	  list of the postcopy blocktime per vCPU.  This is only present
    # 	  when the postcopy-blocktime migration capability is
    # 	  enabled. (Since 3.0)

However, I'd rather have

    # @postcopy-vcpu-blocktime: list of the postcopy blocktime per vCPU.
    #     This is only present when the postcopy-blocktime migration
    #     capability is enabled.  (Since 3.0)

because this is how rST field and option lists work.

To get this, we need to let the first non-blank line after the
"@name:" line determine expected indentation.

This fills up the indentation pitfall mentioned in
docs/devel/qapi-code-gen.rst.  A related pitfall still exists.  Update
the text to show it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-14-armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[Work around lack of walrus operator in Python 3.7 and older]