bug#25626: 25.1; doc of `bufferpos-to-filepos' for type `exact'

[Drew Adams]

> > > This text simply says that this option might be computationally
> > > expensive.
> >
> > If that's the intent then say that.  And I think it is the
> > intent, hence this bug: please just say that.
> 
> Done.
> 
> > [There are also typos in the doc string of `encode-coding-region':
> > "Optional 4th arguments" should be "Optional 4th argument" and
> > "is replace by" should be "is replaced by".]
> 
> Fixed.

Thank you.