C++: Add note to the .qhelp.

Author: geoffw0

cpp/ql/src/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.qhelp

@@ -14,6 +14,9 @@ function may behave unpredictably.</p>
 <p>This may indicate a misspelled function name, or that the required header containing
 the function declaration has not been included.</p>
 
+<p>Note: This query is not compatible with build mode: none databases, and produces
+no results on those databases.</p>
+
 </overview>
 <recommendation>
 <p>Provide an explicit declaration of the function before invoking it.</p>
@@ -26,4 +29,4 @@ the function declaration has not been included.</p>
 <references>
 <li>SEI CERT C Coding Standard: <a href="https://wiki.sei.cmu.edu/confluence/display/c/DCL31-C.+Declare+identifiers+before+using+them">DCL31-C. Declare identifiers before using them</a></li>
 </references>
-</qhelp>
+</qhelp>