[structure.summary] Remove obsolete paragraph about Note(s): and Example(s): elements that we no longer provide

Also restyle a manual Note: in [unord.req] to use the standard note environment.

Author: tkoeppe

source/lib-intro.tex

@@ -401,10 +401,6 @@
 Each subclause also provides a summary, listing the headers specified in the
 subclause and the library entities provided in each header.
 
-\pnum
-Paragraphs labeled ``Note(s):'' or ``Example(s):'' are informative, other paragraphs
-are normative.
-
 \pnum
 The contents of the summary and the detailed specifications include: