admin.rst: add information about maxmem/minmem

- Specify how maxmem and minmem are used at startup, failover and
  migrate
- Add a paragraph about how to manually change the runtime memory, and
  what Ganeti will and won't do automatically as of version 2.6

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>