Notation of `is_file`, `is_dir `… in the manual

Several functions like `is_file`, `is_dir` and others are documented in
chapter 9 of the manual:

http://pike.lysator.liu.se/docs/man/chapter_9.html#9.1

But when I tried them they were not recognized.  Finally I found out
they are defined in the Stdio module:

http://pike.lysator.liu.se/generated/manual/modref/ex/predef_3A_3A/Stdio.html#Stdio

It was confusing because other functions and types documented in chapter
9 do include the `Stdio` module prefix.

-- 
Marcos Cruz
http://programandala.net