Var as "see also" module qualifier in BIPS text docs

Hi,

It seems a standard idiom in the "see also" section of predicate
documentation to use `_' as a module qualifier for a predicate with a
generic name.  The HTML documentation handles this by generating one "see
also" for each module which defines this predicate.  However, the text
documentation just leaves it as an unbound variable, which leads to entries
like `_5837 : =:= / 2' appearing in the documentation.

Cheers,
Warwick