[xmlsec] Extra documentation patch

[Aleksey Sanin]

Hi, Wouter!

>This patch adds the missing documentation in the source from this list:
>  
>
I applied your patch and after few trivial tweaks to make docbook happy
everything looks great! xmlsec-mscrypto now has 100% docs coverage!

>Also I've commented 1 error msg in src/mscrypto/keysstore.c, since
>failing the function CertStrToName isn't really an error, since it is
>not per se a subject dn that is expected there. A conversion fail simply
>means we don't have a dn, and we can stop for the search there.
>
Yes, this make sense. I've also noticed that there is next CertStrToName 
that
was also cause an error when it was actually not an error. I changed 
that too
(hope I did not make a mistake :) ) . And while I was there I have 
created xmlSecMSCryptoCertStrToName
wrapper to simplify the code (I thought to do it later anyway :) ).

Everything is checked in and all tests expected to succeed do succeed :)

Thanks a lot again!

Aleksey