doc/styleguide: establish _comp_ prefix for our functions

doc/styleguide.txt

@@ -118,6 +118,20 @@ name typically based on array name but in singular when looping over actual
 values. If an index or value is to be accessed later on instead of being just
 locally for looping, use a more descriptive and specific name for it.
 
+Function names
+--------------
+
+Use the _comp_ prefix for all function names, and _comp_cmd_ for functions
+defined in per command completion files and not anywhere else. Prefixing with
+an underscore helps keep the functions out of the way for most command name
+completions (except obviously ones starting with an underscore or ones that have
+nothing typed in yet), and having a consistent prefix helps avoid some clashes
+and gives a hint where a function originates from.
+
+It is known that a lot of functions in the tree do not follow this practice.
+This is due to backwards compatibility reasons, but all functions introduced
+after version 2.11 should follow this name prefix rule.
+
 /////////////////////////////////////////
 case/esac vs if
 ---------------
@@ -128,7 +142,7 @@ quoting
 awk vs cut for simple cases
 ---------------------------
 
-variable and function naming
-----------------------------
+variable naming
+---------------
 
 /////////////////////////////////////////