qapi: Improve migration TLS documentation

qapi: Improve migration TLS documentation

MigrateSetParameters is about setting parameters, and
MigrationParameters is about querying them.  Their documentation of
@tls-creds and @tls-hostname has residual damage from a failed attempt
at de-duplicating them (see commit de63ab61241 "migrate: Share common
MigrationParameters struct" and commit 1bda8b3c695 "migration: Unshare
MigrationParameters struct for now").

MigrateSetParameters documentation issues:

* It claims plain text mode "was reported by omitting tls-creds"
  before 2.9.  MigrateSetParameters is not used for reporting, so this
  is misleading.  Delete.

* It similarly claims hostname defaulting to migration URI "was
  reported by omitting tls-hostname" before 2.9.  Delete as well.

Rephrase the remaining @tls-hostname contents for clarity.

Enum MigrationParameter mirrors the members of struct
MigrateSetParameters.  Differences to MigrateSetParameters's member
documentation are pointless.  Copy the new text to MigrationParameter.

MigrationParameters documentation issues:

* @tls-creds runs the two last sentences together without punctuation.
  Fix that.

* Much of the contents on @tls-hostname only applies to setting
  parameters, resulting in confusion.  Replace by a suitable abridged
  version of the new MigrateSetParameters text, and a note on
  @tls-hostname omission in 2.8.

Additional damage is due to flawed doc fix commit
66fcb9d651d (qapi/migration: Add missing tls-authz documentation):
since it copied the missing MigrateSetParameters text from
MigrationParameters instead of MigrationParameter, the part on
recreating @tls-authz on the fly is missing.  Copy that, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240322135117.195489-2-armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
[Some typos corrected]