docs: uctypes: Describe how to instantiate struct objects.

docs/library/uctypes.rst

@@ -98,8 +98,8 @@ Module contents
 
 .. class:: struct(descriptor, layout_type)
 
-   Create a "foreign data structure" object based on its descriptor (encoded
-   as a dictionary) and layout type.
+   Create a "foreign data structure" class based on its descriptor (encoded
+   as a dictionary) and layout type (see below).
 
 .. data:: LITTLE_ENDIAN
 
@@ -140,6 +140,22 @@ Module contents
    so it can be both written too, and you will access current value
    at the given memory address.
 
+Structure classes and instantiating structure objects
+-----------------------------------------------------
+
+Given structure descriptor and layout type, you can instantiate a
+"structure class" using uctypes.struct() factory function. From it,
+you can instantiate a specific structure instance at a given
+memory address. Memory address usually comes from following sources:
+
+* Predefined address, when accessing hardware registers on a baremetal
+port. Lookup these addresses in datasheet for a particular MCU/SoC.
+* As return value from a call to some FFI (Foreign Function Interface)
+function.
+* From uctypes.addressof(), when you want to pass arguments to FFI
+function, or alternatively, to access some data for I/O (for example,
+data read from file or network socket).
+
 Structure objects
 -----------------