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

[Eli Zaretskii]

> Date: Sun, 5 Feb 2017 10:58:24 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 25626@debbugs.gnu.org
> 
> > 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.