[office-formula] IMLOG, IMLOG10, IMLOG2 and IMSQRT have no formula

Hi members,

the definitions of the functions IMLOG, IMLOG10, IMLOG2 and IMSQRT are 
weak and lack formulas.

For example:
IMLN
Summary: Returns the natural logarithm of a complex number.
Semantics: If X=r(cos φ + i sin φ) , φ is expressed by radians, then the 
natural logarithm is returned.

That is a tautology.

In general logarithm and roots are multivalued functions for complex 
values. Therefore the definition should explicit say, which value is 
returned. If there is a standard which defines the principal value (I 
don't know) then of cause a reference to that "principal value" would be 
enough.

kind regards
Regina