Proposed rewording of filter tooltips

For #16186. Proposed changes to the tooltips which appear when a filter
expression is potentially problematic.

Rename references to "User's Guide" to "Help" since the link to the
User's Guide in the Help menu is just called *Contents*.

Name specific sections within the help which pertain to the warning
tooltip being shown. Gives first-time users some help in finding the
right part of the sizeable User's Guide.

ui/qt/widgets/syntax_line_edit.cpp

@@ -196,10 +196,10 @@ bool SyntaxLineEdit::checkDisplayFilter(QString filter)
             header_field_info *hfi = proto_registrar_get_byalias(token_str);
             if (hfi)
                 syntax_error_message_ = tr("\"%1\" is deprecated in favour of \"%2\". "
-                                           "See the User's Guide.").arg(token_str).arg(hfi->abbrev);
+                                           "See Help section 6.4.9 for details.").arg(token_str).arg(hfi->abbrev);
             else
                 syntax_error_message_ = tr("\"%1\" may have unexpected results. "
-                                           "See the User's Guide.").arg(token_str);
+                                           "See Help section 6.4.8 for details.").arg(token_str);
             g_free(token_str);
         } else {
             setSyntaxState(SyntaxLineEdit::Valid);