add generators.function docs & examples (#437)

garak/generators/function.py

@@ -3,6 +3,31 @@
 
 Call a given function to use as a generator; specify this as either the 
 model name on the command line, or as the parameter to the constructor.
+
+This generator is designed to be used programmatically, rather than 
+invoked from the CLI. An example usage might be:
+
+```
+import mymodule
+import garak.generators.function
+
+g = garak.generators.function.Single(name="mymodule#myfunction")
+```
+
+The target function is expected to take a string, and return a string.
+Other arguments passed by garak are forwarded to the target function.
+
+Note that one can import the intended target module into scope and then
+invoke a garak run via garak's cli module, using something like:
+
+```
+import garak
+import garak.cli
+import mymodule
+
+garak.cli.run("--model_type function --model_name mymodule#function_name --probes encoding.InjectBase32".split())
+```
+
 """