Add reference to parseint() in conversion docs

Nicer for the user who may wonder what's the correct solution.

doc/manual/conversion-and-promotion.rst

@@ -105,6 +105,8 @@ numbers as strings to be conversions (many dynamic languages will even
 perform conversion for you automatically), however Julia does not: even
 though some strings can be parsed as numbers, most strings are not valid
 representations of numbers, and only a very limited subset of them are.
+Therefore in Julia the dedicated ``parseint`` function must be used
+to perform this operation, making it more explicit.
 
 Defining New Conversions
 ~~~~~~~~~~~~~~~~~~~~~~~~