libc: fix description issues in mac_text(3)/mac_free(3)

mac_text(3) as-written would seem to indicate that a mac_t should be
freed with free(3), but this isn't the case. One can derive from
context from when the change was introduced and COMPATIBILITY that this
was intended to talk about *text in mac_to_text, so move the comment
there.

PR: 179832
Co-authored-by: Priit Järv <priit cc ttu ee>
MFC after: 3 days