doc: refs: put ref migration warning under the command
I have to scroll down at least three screens in man(1) from the `migrate` description in order to see the “known limitations” for it. This is important information since the text says that concurrent writes can lead to an inconsistent migrated state. Let’s move that text up to the command description and put it inside a Caution admonition. This section made sense when it was added in 25a0023f (builtin/refs: new command to migrate ref storage formats, 2024-06-06); `migrate` was the only subcommand, and this section was visible from the command description. A one-page man page. But that is not the case anymore now that the command has nine subcommands to describe. Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>