If statements and unused template parameters in Phobos documentation

[Rekel via Digitalmars-d-learn]

I found a lot of the Phobos documentation to contain template 
arguments and if statements that made no sense to me, for example:

```
 uint readf(alias format, A...) (
 auto ref A args
)
if (isSomeString!(typeof(format)));
uint readf(A...) (
 scope const(char)[] format,
 auto ref A args
);
``` https://dlang.org/library/std/stdio/file.readf.html

From stdio.readf & stdio.File.readf. I'm assuming this is some 
kind of template, but often it seems there are more parameters in 
the first '()' part than are ever given. Am I missing something? 
Additionally, what is that if statement for? It precedes nothing.