migration/cpr: Document obscure usage of g_autofree when parse str

migration/cpr: Document obscure usage of g_autofree when parse str

HMP parsing of cpr_exec_command contains an obscure usage of g_autofree.
Provide a document for it to be clear that it's intentional, rather than
memory leaked.

Cc: Dr. David Alan Gilbert <dave@treblig.org>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
Link: https://lore.kernel.org/r/20251023161657.2821652-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>