Changed ruleset for Enum types.

The ruleset for enum definition "shall" is not useful in many cases where "_UNKNOWN_" and "_OTHER_" is more confusing than helpful. So  it should only be a recommendation for future implmentations.

Signed-off-by: Thomas Hempen <thomas.hempen@thi.de>

Author: thempen

content/contributing/naming_conventions.adoc

@@ -43,8 +43,8 @@ message EnvironmentalConditions
 
 Enum field names shall use _UPPER_SNAKE_CASE_.
 Enum fields shall always be prefixed with the name of the corresponding enum.
-The first enum field shall always use the suffix _UNKNOWN_.
-The second enum field shall always use the suffix _OTHER_.
+The first enum field should always use the suffix _UNKNOWN_.
+The second enum field should always use the suffix _OTHER_.
 
 [source,protobuf,linenums]
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