asciidoc adds a timestamp at the end of a generated HTML file.
This timestamp is based on the date of the file but it can change
depending on the TZ environment variable.
Previously, we said (more or less), "a2x is broken and here's how you could
try to fix it". Instead, we now say "We need a2x to build manpages; a2x
didn't work; here is a fix that might work for you; alternatively you
could just skip manpage building."
Addresses bug 1524.
Also, give the message as a here-document rather than a bunch of echos.
Zax from #tor noticed that while we list docbook-xsl and docbook-xml
in our helpful error text when making the docs fails, we forgot to
also list libxml2-utils. Let's add that.
