[spec] Multiple substitution with classes syntax

[khaledhosny]

Description

Document syntax for using glyph classes in multiple substitution rules. The syntax used by GlyphsApp, and recently also supported by feaLib.

Checklist:

• I have followed the Contribution Guidelines
• I have added test code and data to prove that my code functions correctly
• I have verified that new and existing tests pass locally with my changes
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

[miguelsousa]

@khaledhosny Actions and other GH systems are experiencing an outage; that's why CI hasn't run yet.

[khaledhosny]

@khaledhosny Actions and other GH systems are experiencing an outage; that's why CI hasn't run yet.

This is only documentation change, though, so probably there is not much for the CI to catch anyway.

[miguelsousa]

I see. Do you mind creating an issue to request supporting the new formats in makeotf?

[khaledhosny]

#1652

[skef]

This looks like a good extension to the syntax and we will likely add it. For now the addition seems lower priority than the ongoing VF-related work, so it will happen later. We generally try to keep the FDK current with the grammar so the doc update will happen at the same time #1652 is resolved.