[latexrefman] calc package

[Vincent Belaïche]

In @node \includegraphics, one can read

An example is that by using the @file{calc} package you can make the
graphic be 1 at tie{}cm narrow than the width of the text with
@code{\includegraphics[width=\linewidth-1.0cm]@{hefferon.jpg@}}.

Several comments:

* shouldn't we use @dmn{…} for units
* narrow -> narrower ? (Not being a native English speaker…)
* missing @PkgIndex{calc}
* \dimexpr can be used with modern engines

If everybody OK, I can implement these changes with my next commit.
  V.