qapi/migration: Don't document MigrationParameter

qapi/migration: Don't document MigrationParameter

The MigrationParameter (singular) enumeration is not part of the
migration QMP API, it's only used for nicely converting HMP strings
into MigrationParameters (plural) members and for providing readline
completion.

Documenting this enum only serves to duplicate documentation between
MigrationParameter and MigrationParameters.

Add an exception to QAPIs pragma.json and stop documenting it.

The generated "QEMU QMP Reference Manual" now lists the enum members
as "Not documented."  Tolerable.

Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251215220041.12657-8-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>