Illustrate CONVENTIONS FOR LOCALIZATION (L10N) with examples

In the course of writing these examples, I thought of two enhancements:

* capitalize first letter of key name to differentiate parameter names
  that refer to a template message from parameter names that refer to
  a different type of value (still starting with a lower-case letter)

* void template blocks by returning the empty string "" for the template
  key instead of returning NULL for a parameter within. This will free
  the NULL value from its special role, allowing its use as default value
  to leave parameters detected as false positive unreplaced.
New convention: use upper-case initial for keys of messages/templates

The different case makes it easier to differentiate references to
templates from dynamic values at first glance.
Add missing word 'a'
Use explicit empty strings to void blocks, use null as default value

The blocks that make parts of templates can now be silenced in a simpler
way by having the application provide an empty string for the key. This
value takes precedence over the template value, which is only looked up
when no value (null) is provided by the application for the key.

This simplifies the complex example greatly, which is shortened by the
fact that there is no need to parse a template that will eventually be
skipped.

The null value is now assigned the role of the default value which allows
to keep a parameter unreplaced, when no matching message is found either.