There was a small window in ipnserver after we assigned a LocalBackend
to the ipnserver's atomic but before we Start'ed it where our
initalization Start could conflict with API calls from the LocalAPI.
Simplify that a bit and lay out the rules in the docs.
Updates #12028
Change-Id: Ic5f5e4861e26340599184e20e308e709edec68b1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>