C-u prefix input (e.g., C-x f / set-fill-column)

[David Reitter]

The documentation for `fill-column' reads:

> Interactively, you can set the buffer local value using C-x f.

When you do C-x f, you get an error message "set-fill-column requires  
an explicit argument".

(The documentation for `set-fill-column' is okay.)

I would propose two changes:

1. The documentation should be

"Interactively, you can set the buffer local value using C-u <VALUE>  
C-x f."


2a. When users forget to give an argument with C-u, the error message  
should be

"set-fill-column requires an explicit argument with prefix C-u  
<ARGUMENT>"

or something like that.
Or, better:

2b. Instead of displaying the above error message, the user should be  
queried interactively for the argument, just like most interactive  
functions do it. In other words, use (interactive "N") in `set-fill- 
column' instead of "p".


FWIW, interactive "p" is used 213 times, but in most situations, the  
prefix seems to be optional, i.e. the change to "N" wouldn't be  
justified.