Add usage of OpenType font feature to the documents (issue 328140043 by address@hidden)

[pkx166h]

https://codereview.appspot.com/328140043/diff/1/Documentation/notation/text.itely
File Documentation/notation/text.itely (right):

https://codereview.appspot.com/328140043/diff/1/Documentation/notation/text.itely#newcode1464
Documentation/notation/text.itely:1464:
Would it be appropriate to add something after this @lilypond example
like:

For the full OpenType font feature list please see:

@uref{https://www.microsoft.com/typography/otspec/featurelist.htm}

Otherwise LGTM

https://codereview.appspot.com/328140043/