[lex.pptoken] Index keyword placeholders (#8367)

Author: jensmaurer

source/lex.tex

@@ -586,6 +586,9 @@
 or an operator or punctuator.
 
 \pnum
+\indexgrammar{\idxgram{import-keyword}}%
+\indexgrammar{\idxgram{module-keyword}}%
+\indexgrammar{\idxgram{export-keyword}}%
 The \grammarterm{import-keyword} is produced
 by processing an \keyword{import} directive\iref{cpp.import},
 the \grammarterm{module-keyword} is produced