Document the quant_warning type's reason for being.

compiler/quantification.m:
    As above.

compiler/make_hlds_warn.m:
    Give the predicates translating quant_warnings into actual error messages
    more meaningful names, and a more rational argument order.

compiler/add_class.m:
compiler/add_clause.m:
    Conform to the above.
Merge branch 'master' into fix-syntax-vim