Properly document IsVoidMatrix, IsInitialMatrix, IsInitialIdentityMatrix, ...

[fingolfin]

Many "documented" properties in homalg actually have no meaningful description; they just say this in their documentation comment:

##    <Returns><C>true</C> or <C>false</C></Returns>
##    <Description>
##      <A>A</A> is a &homalg; matrix.
##    </Description>

That's not helpful in the least. Sure, for some of them, one can guess, but I have e.g. no idea what IsVoidMatrix means.