Doc: Add note about fromPercentEncoding's behavior on invalid input

Add a note saying that invalid input to QByteArray::fromPercentEncoding
and QUrl::fromPercentEncoding will produce invalid output, and provide
an example.

Change-Id: Icc68f59c23cf199640b646cd4a6ca8e4808a3f71
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qurl.cpp

@@ -3440,6 +3440,10 @@ QUrl QUrl::fromEncoded(const QByteArray &input, ParsingMode mode)
 /*!
     Returns a decoded copy of \a input. \a input is first decoded from
     percent encoding, then converted from UTF-8 to unicode.
+
+    \note Given invalid input (such as a string containing the sequence "%G5",
+    which is not a valid hexadecimal number) the output will be invalid as
+    well. As an example: the sequence "%G5" could be decoded to 'W'.
 */
 QString QUrl::fromPercentEncoding(const QByteArray &input)
 {

src/corelib/tools/qbytearray.cpp

@@ -4484,6 +4484,10 @@ void q_fromPercentEncoding(QByteArray *ba)
         text.data();            // returns "Qt is great!"
     \endcode
 
+    \note Given invalid input (such as a string containing the sequence "%G5",
+    which is not a valid hexadecimal number) the output will be invalid as
+    well. As an example: the sequence "%G5" could be decoded to 'W'.
+
     \sa toPercentEncoding(), QUrl::fromPercentEncoding()
 */
 QByteArray QByteArray::fromPercentEncoding(const QByteArray &input, char percent)