Clarify that text in dateTimeFormat is part of a date/time pattern

[pedberg]

UTS35 should clarify that (aside from the substitution markers {0} and {1}) text is a dateTimeFormat is part of a date/time pattern and is subject to the same rules described in Appendix F: Date Format Patterns ​http://unicode.org/repos/cldr/trunk/specs/ldml/tr35.html#Date_Format_Patterns, including quoting of ASCII-range characters if they are to be interpreted as literal text, and use of 2 successive APOSTROPHE characters to represent a single APOSTROPHE.