Document the `fmt` filter

[René Kijewski]

Looking at the generator code I cannot tell, and it is not documented in the book. The code ensures that no more than 2 arguments are given (the first argument being the left-hand side of the pipe), but the second argument in …|fmt(second) is never used anyway.

Is this filter simply a broken copy of …|format?

[René Kijewski]

Ah, git blame told me it's related to #345. Also, I did not read the code properly, the argument is used.

[Dirkjan Ochtman]

So any questions/concerns left here?

[René Kijewski]

Yeah, the documentation is still missing.