[structure.requirements] Add paragraph explaining the typographical convention for requirement names

Author: CaseyCarter

source/lib-intro.tex

@@ -483,6 +483,18 @@
 well-defined expression requirements is described in terms of the valid expressions for
 its template type parameters.
 
+\pnum
+The library specification uses a typographical convention for naming
+requirements. Names in \textit{italic} type that begin with the prefix
+\oldconcept{} refer to sets of well-defined expression requirements typically
+presented in tabular form, possibly with additional prose semantic requirements.
+For example, \oldconcept{Destructible}~(\tref{destructible}) is such a named
+requirement. Names in \tcode{constant width} type refer to library concepts
+which are presented as a concept definition\iref{temp}, possibly with additional
+prose semantic requirements. For example,
+\libconcept{Destructible}\iref{concept.destructible}
+is such a named requirement.
+
 \pnum
 Template argument requirements are sometimes referenced by name.
 See~\ref{type.descriptions}.