Opentype features now enabled? Documentation?

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It is LibreOffice 5.3 which brings OpenType feature support. See the "Text Layout" section of the LibreOffice 5.3 release notes. "OpenType layout features can be controlled using the syntax previously only supported for Graphite fonts."

Here's a summary of the syntax:

1. OpenType features are activated by add special codes to the end of the font name.
2. Where you have UI that you can select a font from a pop-up menu, you likely can also type the font name. Two useful locations are:
   • the Font Name area in the Formatting toolbar,
   • the three Font Name selectors in the Character formatting dialogue, reached via menu item Format… Character…, then selecting the Font tab
   • the three Font Name selectors in the Paragraph Style formatting dialogue, reached via menu item Styles and Formatting, then right-clicking on a Style name, selecting Modify… from the popup menu,then selecting the Font tab
   • the Font Name area in the Properties sidebar,
3. In those locations, select the font name, then click in the font name. A cursor appears. Use right-arrow to move to the end of the font name. Type ":" and the first code. Click OK or however you commit the change. The document will redraw, with features applied.
4. The code consists of a 4-character "tag", with notation before or after it. Just the tag means "activate this feature", e.g. onum activates old-style numbers. A minus in front of the tag means "deactivate this feature". The tag followed by "=" then an integer means "assign the integer value to the tag for layout purposes", e.g. frac=1 enables arbitrary (diagonal) fractions, while frac=2 enables precomposed (nut) fractions.
5. You can add multiple codes after the first code. To add a code, insert "&" as a separator, then the next code. e.g. :onum=1&pnum=1 activates both the Oldstyle Figures and Proportional Figures features.
6. The authoritative list of OpenType feature tags is at https://www.microsoft.com/typography/... . Some fonts may define their own feature tags using the OpenType or Graphite mechanisms, e.g. http://www.numbertext.org/linux/fontf... . You might find that trying to figure out which fonts work with which tags on which versions of LO will be a bit of a mess. Sorry.
7. Be aware that OpenType, and Graphite, and Apple Advanced Typography (AAT) are three different systems for defining how an application like LibreOffice should interact with a font to make text layout happen. They share common font structures, but the tag names and expected behaviour are different. This makes things more complicated. Sorry.

(Note to reader: I probably left something out of this, or stated it wrongly. Please improve this answer as we all learn more.)