README.links to explain xref properly.

Block elements with verbatim formatting (literallayout, programlisting,
screen, synopsis) should be aligned at column 0 independent of the surrounding
SGML, because whitespace is significant, and indenting them creates erratic
whitespace in the output.  The CSS stylesheets already take care of indenting
the output.

Assorted markup improvements to go along with it.

Sergey Karpov

Fix lots of bad markup, bad English, bad explanations.

This commit covers only about half the contrib modules, but I grow weary...

Albert Cervera i Areny