Fix to_date* functions links to qt doc

QDateTime::fromString documentation no longer lists all the expressions
to construct the format, so we point to each component doc

resources/function_help/json/to_date

@@ -2,7 +2,7 @@
   "name": "to_date",
   "type": "function",
   "groups": ["Conversions", "Date and Time"],
-  "description": "Converts a string into a date object. An optional format string can be provided to parse the string; see <a href='https://doc.qt.io/qt-5/qdate.html#fromString-1'>QDate::fromString</a> for additional documentation on the format.",
+  "description": "Converts a string into a date object. An optional format string can be provided to parse the string; see <a href='https://doc.qt.io/qt-5/qdate.html#fromString-2'>QDate::fromString</a> for additional documentation on the format.",
   "arguments": [
    {"arg":"string","description":"string representing a date value"},
    {"arg":"format","optional":true,"description":"format used to convert the string into a date"},

resources/function_help/json/to_datetime

@@ -2,7 +2,7 @@
   "name": "to_datetime",
   "type": "function",
   "groups": ["Conversions", "Date and Time"],
-  "description": "Converts a string into a datetime object. An optional format string can be provided to parse the string; see <a href='https://doc.qt.io/qt-5/qdatetime.html#fromString-1'>QDateTime::fromString</a> for additional documentation on the format.",
+  "description": "Converts a string into a datetime object. An optional format string can be provided to parse the string; see <a href='https://doc.qt.io/qt-5/qdate.html#fromString-2'>QDate::fromString</a> and <a href='https://doc.qt.io/qt-5/qtime.html#fromString-1'>QTime::fromString</a> for additional documentation on the format.",
   "arguments": [
    {"arg":"string","description":"string representing a datetime value"},
    {"arg":"format","optional":true,"description":"format used to convert the string into a datetime"},