doc updates (2) (op backannotation)

doc/xschem_man/tutorial_ngspice_backannotation.html

@@ -144,8 +144,13 @@ write cmos_example.raw
   calculated as <kbd>V(GN) * Id(m3)</kbd>.
  </p>
  <img src="backannotation12.png">
+ <p> you can wrap the whole expression inside a <kbd>[to_eng ... ]</kbd> to have the value displayed inengineering form
+  using the usual SPICE suffixes (example: 131u for 131e-6)
+ </p>
+ <img src="backannotation15.png">
+
  <p> 
-  The example shown uses this component to display a (meaningless, but shows the usage) gm ratio of 2 transistors:
+  The example shown below uses this component to display a (meaningless, but shows the usage) gm ratio of 2 transistors:
  </p>
  <img src="backannotation13.png">
  <p> 

xschem_library/examples/cmos_example.sch

@@ -1589,3 +1589,6 @@ C {ngspice_get_value.sym} 520 -310 0 0 {name=r6 node=i(@$\{path\}m4[id])
 descr="I="}
 C {ngspice_get_value.sym} 670 -310 0 1 {name=r7 node=i(@$\{path\}m5[id])
 descr="I="}
+C {ngspice_get_expr.sym} 160 -160 0 0 {name=r8 node="[expr \{[ngspice::get_voltage gn] * [ngspice::get_current \{m3[id]\}]\}]"
+descr="Power="
+}