My paper copy of K&R1 (1978) does not mention void, nor does it mention standard headers other than stdio.h. The linked document may be a later draft (and it's almost certainly a copyright violation).
Prior to C99, reaching the closing "}" of main() caused the program to return an undefined status to the calling environment. In C99 and later (as in C++), reaching the closing "}" does an implicit "return 0;".
I had some problems in believing that C99 says that main() implicitly returns 0, so for future reference: It says so in 5.1.2.2.3, even including wording concerning the closing "}" (which seems to me as slightly quirky way to describe what it is describing).
As for stdio.h being only header: it makes sense, as headers like stdlib.h and string.h contain mostly function declarations, which you don't strictly need in K&R. Some of 80's UNIX C code examples in books I've seen don't include anything (and there are even some that declare things like FILE and errno directly without including it from anywhere).
Prior to C99, reaching the closing "}" of main() caused the program to return an undefined status to the calling environment. In C99 and later (as in C++), reaching the closing "}" does an implicit "return 0;".