bug#12344: 24.2.50; Typo in "This variable is safe as a file local..."

[Drew Adams]

> This help text seems to be missing a word or two or three:
>  
>   This variable is safe as a file local variable if its value
>   satisfies the predicate which is byte-compiled expression.

I see now that this text is only for `C-h v byte-compile-warnings', and that the
last part of the printed text varies, depending on the particular variable.

Please fix this for at least variable `byte-compile-warnings'.  Clearly, the
general pattern/algorithm used to come up with the text for any given variable
is flawed.  Or else the data it is using to come up with the text is flawed in
the case of `byte-compile-warnings'.