add C printf function details link

[Dan Jacobson]

The stat(1) docs don't mention that -c FORMATS are printf(3) style
formats and there is no reference to click to learn about them.

The find(1) -printf docs say "Unlike the C `printf' function, they do
not support field width specifiers." which is wrong,
$ find z -printf %-5fq\\n
z    q
therefore add a reference there to the "C printf function", probably
man 3 printf.