doc:  mention <link> can be inside of <command>, but not <xref>
This was discussed in commit
Reported-by: Peter Eisentraut
Discussion:
@@ -22,6 +22,7 @@ endterm=
<link>
use to supply text for the link, only uses linkend, requires </link>
http://www.oasis-open.org/docbook/documentation/reference/html/link.html
+ can be embedded inside of <command>, unlike <xref>
External Linking
