[intro.refs], [intro.defs] Update introductory text to match latest ISO

Directives.

Fixes #1499.

Author: zygoloid

source/intro.tex

@@ -26,7 +26,9 @@
 
 \pnum
 \indextext{references!normative|(}%
-The following referenced documents are indispensable for the application
+The following documents are referred to in the text
+in such a way that some or all of their content
+constitutes requirements
 of this document. For dated references, only the edition cited applies.
 For undated references, the latest edition of the referenced document
 (including any amendments) applies.
@@ -80,10 +82,19 @@
 \indextext{definitions|(}%
 For the purposes of this document,
 the terms and definitions
-given in ISO/IEC 2382-1:1993 and
+given in ISO/IEC 2382-1:1993,
 the terms, definitions, and symbols
-given in ISO 80000-2:2009 apply,
-as do the following definitions.
+given in ISO 80000-2:2009,
+and the following apply.
+
+\pnum
+ISO and IEC maintain terminological databases
+for use in standardization
+at the following addresses:
+\begin{itemize}
+\item IEC Electropedia: available at \url{http://www.electropedia.org/}
+\item ISO Online browsing platform: available at \url{http://www.iso.org/obp}
+\end{itemize}
 
 \pnum
 \ref{definitions}