[intro.defs] Fix introductory text according to ISO rules.

In principle, "symbols and abbreviated terms" can be listed in a
standard, and can be listed in a separate clause or in a combined
clause 3 "Terms, definitions, symbols and abbreviated terms", we do
not actually need symbol definitions. In any case, the introductory
text would never mention "symbols".

Author: tkoeppe

source/intro.tex

@@ -76,9 +76,7 @@
 \indextext{definitions|(}%
 For the purposes of this document,
 the terms and definitions
-given in ISO/IEC 2382,
-the terms, definitions, and symbols
-given in ISO 80000-2:2009,
+given in ISO/IEC 2382 and ISO 80000-2:2009,
 and the following apply.
 
 \pnum