Ticket 363: Strengthen string->number

Peter Bex writes (slightly edited):

Improve the wording in the description of string->number from:

"If string is not a syntactically valid notation for a number, string->number returns #f."

to something like:

"If string is not a syntactically valid notation for a number, or would result in a number that the implementation cannot represent, string->number returns #f."

I'd also like to suggest adding this line to the end:

"The string->number procedure always returns a number or #f; it never raises an exception due to the content of the string."