Preprocessor. Z80 improvements. Library improvements.

docs/api/command-line.md

@@ -34,6 +34,7 @@ no extension for BBC micro program file,
 
 * `-r <program>` – Run given program after successful compilation. Useful for automatically launching emulators without any external scripting.
 
+* `-D <feature>=<value>` – Defines a feature value for the preprocessor.
 
 ## Verbosity options
 

docs/api/custom-platform.md

@@ -64,6 +64,10 @@ Default: the same as `encoding`.
     
     * `lenient_encoding` - allow for automatic substitution of invalid characters in string literals using the default encodings, default is `false`.
 
+#### `[define]` section
+
+This section defines values of features of the target.
+See the [preprocessor documentation](../lang/preprocessor.md) for more info.
 
 #### `[allocation]` section