qapi-block-core: Clean up blockdev-snapshot-internal-sync doc

qapi-block-core: Clean up blockdev-snapshot-internal-sync doc

BlockdevSnapshotInternal is the arguments type of command
blockdev-snapshot-internal-sync.  Its doc comment contains this note:

    # .. note:: In a transaction, if @name is empty or any snapshot matching
    #    @name exists, the operation will fail.  Only some image formats
    #    support it; for example, qcow2, and rbd.

"In a transaction" is misleading, and "if @name is empty or any
snapshot matching @name exists, the operation will fail" is redundant
with the command's Errors documentation.  Drop.

The remainder is fine.  Move it to the command's doc comment, where it
is more prominently visible, with a slight rephrasing for clarity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240718123609.3063055-1-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>