[R6RS] issues in document/lex.tex

[Michael Sperber]

I've replaced the S-expression/datum terminology as discussed
yesterday.  I also tried to be more precise where I saw the text was
potentially confusing.  I'd appreciate if somebody could re-review the
relevant section in struc.tex and lex.tex.  (The changes are almost
exclusively local to sentences previously containing the word
"S-expression.")

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla