descriptions: Updated cmake DESCRIPTION field

This commit touches a subset of UPM sensors which contain C source.
Cleaned up the CMakeLists.txt DESCRIPTION field to better represent
the library.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/led/CMakeLists.txt

@@ -1,5 +1,5 @@
 upm_mixed_module_init (NAME led
-    DESCRIPTION "upm led module"
+    DESCRIPTION "LED module"
     C_HDR led.h
     C_SRC led.c
     CPP_HDR led.hpp

src/led/led.hpp

@@ -50,8 +50,8 @@ namespace upm {
  * @brief API for the LED
  *
  * UPM module for the LED (or other similar light-emitting diodes).
- * An LED is a small lightbulb that emits light in
- * response to a small current. The longer wire of an LED connects
+ * An LED is a p-n junction semiconductor which emits light in
+ * response to voltage. The longer wire of an LED connects
  * to the positive seat (anode); the shorter wire connects to the
  * negative seat (cathode). The flat side of the bulb corresponds
  * to the cathode, while the rounded side corresponds to the anode.