Clarify variable capitalization rules

notes/style.md

 
 ### variables ###
 
-Common advice applies to variable naming, but capitalization is decided
-based on scope:
+There are three types of variables recognized in terms of this style.
+The distinction is based on scope:
 
  *  Global variables must be `ALL_CAPS`.
 
- *  Local variables must be `lowercase` or `snake_case`, and *must not*
-    be used in inherited scope, i.e. in a child function.
+ *  Function-local variables must be `lowercase` or `snake_case`.
+    These variables *must not* be used in inherited scope, i.e. in
+    a child function.
 
- *  Optionally an exception from the previous rule can be granted to
-    a specific variable.  However, such variable must be named using
-    `CamelCase`.
+ *  Inheritable-local variables must be `CamelCase`.  These variables
+    may be used in inherited scope, i.e. in a child function.
 
  *  Both kinds of local variables (inherited and non-inherited) should
     be defined in header of the function, that is, before actual code.