man spamd says:

       -m number , --max-children=number
           This option specifies the maximum number of children to spawn.  Spamd will spawn that number of children, then sleep in the
           background until a child dies, wherein it will go and spawn a new child.


OK, but setting this to 1 will cause logs to have

    Thu Jan 24 22:42:55 2019 [18861] info: prefork: server reached --max-children setting, consider raising it

with no way to turn off the message. What if the user always processes
his spam one by one serially, not in parallel? So please give him a way
to disable this particular warning.

Also mention this is affected by --max-spare! Else people will wonder
why the default (5) is never reached for them.

OK, mention maybe setting --max-spare=1 is the right way us serial users
should do this.

       --min-children=number
           The minimum number of children that will be kept running.  The minimum value is 1, the default value is 1.  If you have lots
           of free RAM, you may want to increase this.

OK but mention what happens if this is set higher than --max-children.

       --min-spare=number
           The lower limit for the number of spare children allowed to run.  A spare, or idle, child is one that is not handling a scan
           request.   If there are too few spare children available, a new server will be started every second or so.  The default value
           is 1.

Also mention what happens if this is set higher than --max-children.

Also mention what setting this to 0 will do.

       --max-spare=number
           The upper limit for the number of spare children allowed to run.  If there are too many spare children, one will be killed
           every second or so until the number of idle children is in the desired range.  The default value is 2.

Also mention what setting this to 0 will do. Will spamd cease to have
any functionality?

       --max-conn-per-child=number
           This option specifies the maximum number of connections each child should process before dying and letting the master spamd
           process spawn a new child.  The minimum value is 1, the default value is 200.

Mention why this is needed. Do more used children get their memory bloated etc.?

OK, on some shared servers, the hosting company may have a rule: no
persistent processes allowed. So another setting missing seems to be:
how long should spamd(s) live waiting around with no messages coming in
to be processed? Currently the user must make his scripts kill it after
sending the last message to it.

As far as an absolute time of life no matter what, mention one can use
"timeout 600 spamd" etc. See timeout(1) man page. But that seems a
rather violent way to kill it, perhaps when it was in the middle of
processing a message.