Reinforce that custom attributes should not be placed in Otel namespaces

Use normative "RECOMMENDED" and add a sentence explicitly reinforcing that
custom attributes don't belong in Otel namespaces.

This was the intent behind the rules and I think we should be allowed to make
this clarification even though the doc is marked "Stable".

specification/common/attribute-naming.md

@@ -122,12 +122,12 @@ To do that consider a few options:
 - The name is specific to your company and may be possibly used outside the
   company as well. To avoid clashes with names introduced by other companies (in
   a distributed system that uses applications from multiple vendors) it is
-  recommended to prefix the new name by your company's reverse domain name, e.g.
+  RECOMMENDED to prefix the new name by your company's reverse domain name, e.g.
   `com.acme.shopname`.
 
 - The name is specific to your application that will be used internally only. If
   you already have an internal company process that helps you to ensure no name
-  clashes happen then feel free to follow it. Otherwise it is recommended to
+  clashes happen then feel free to follow it. Otherwise it is RECOMMENDED to
   prefix the attribute name by your application name, provided that
   the application name is reasonably unique within your organization (e.g.
   `myuniquemapapp.longitude` is likely fine). Make sure the application name
@@ -137,6 +137,9 @@ To do that consider a few options:
   case consider submitting a proposal to this specification to add a new name to
   the semantic conventions, and if necessary also to add a new namespace.
 
+You SHOULD NOT add new attribute names to any OpenTelemetry namespace and use them
+in your telemetry unless the attribute is officially added to the semantic conventions.
+
 It is recommended to limit names to printable Basic Latin characters
 (more precisely to
 [U+0021 .. U+007E](https://en.wikipedia.org/wiki/Basic_Latin_(Unicode_block)#Table_of_characters)