String changes in GCalctool (new manual)

[Robert Ancell <robert ancell gmail com>]

Hi all,

I've replaced the existing GCalctool manual (out of date) with a new
Mallard version.  Sorry for the late arrival, if you find any problems
or have any suggested improvements please commit those directly or
email me with changes.

Additionally there is a string change for the error message displayed
when passing an undefined value to tangent.

--Robert