QFlags::testFlag: clarify the behavior in the docs

The name made me fall in a trap: I thought that testFlag returned
true if *any* of the bits in the argument were set in the QFlags
object. In reality, it returns true if *all* the bits are set, and
an argument equivalent to 0 is treated specially. Clarify this
in the documentation.

Change-Id: I9e8088c48038b4e6ba5830fdb7e473c6d7884b29
Pick-to: 5.15 6.0 6.1
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -501,6 +501,14 @@ static_assert(sizeof(qint64) == 8, "Internal error, qint64 is misdefined");
     \since 4.2
 
     Returns \c true if the flag \a flag is set, otherwise \c false.
+
+    \note if \a flag contains multiple bits set to 1 (for instance, if
+    it's an enumerator equal to the bitwise-OR of other enumerators)
+    then this function will return \c true if and only if all the bits
+    are set in this flags object. On the other hand, if \a flag contains
+    no bits set to 1 (that is, its value as a integer is 0), then this
+    function will return \c true if and only if this flags object also
+    has no bits set to 1.
 */
 
 /*!