Use domain_search_flags in lookup_symbol et al

This changes lookup_symbol and associated APIs to accept
domain_search_flags rather than a domain_enum.

Note that this introduces some new constants to Python and Guile.  I
chose to break out the documentation patch for this, because the
internals here do not change until a later patch, and it seemed
simpler to patch the docs just once, rather than twice.

gdb/f-lang.c

@@ -1715,7 +1715,7 @@ f_language::search_name_hash (const char *name) const
 struct block_symbol
 f_language::lookup_symbol_nonlocal (const char *name,
 				    const struct block *block,
-				    const domain_enum domain) const
+				    const domain_search_flags domain) const
 {
   return cp_lookup_symbol_nonlocal (this, name, block, domain);
 }