Generated ghc man page missing xrefs

[kaol]

The man page for ghc is generated from xml files. Currently, this ignores xrefs, turning, for example

<entry>enable almost all warnings (details in <xref linkend="options-sanity"/>)</entry>

to

enable almost all warnings (details in )

This leads to stunted sentences as well as missing information.

[igloo]

There have been discussions about rewriting the docs in something like markdown. That might be the best way to fix this.