Add information about units of synaptic weight and distance to the units page

[skip ci]

Author: Andrew Davison

doc/units.txt

@@ -16,4 +16,7 @@ conductance       µS
 capacitance       nF
 firing rate       /s
 phase/angle       deg
-================= =====
+================= =====
+
+Synaptic weights are in microsiemens or nanoamps, depending on whether the post-synaptic mechanism implements a change in conductance or current.
+Distances are typically in microns, but any distance scale can be used, provided it is used consistently.