doc: refs: put ref migration warning under the command

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 Warning 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.

Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>