partitions.conf uses magic values for rate and mtu which are not documented

[KelleyCook]

And experienced IB user know they need to add mtu=5 to their partitions.conf in order enable MTU size of 4096. What 5 means is buried in the specification and/or include files.

This patch has two related changes to the partitions.conf doc.
First, make tables of the magic values.
Secondly, change the example so one would be able to find it by searching for 4096 as its not useful to have the example be the default 2K.

[KelleyCook]

See Pull Request #32 which I thought would have auto-attached