Linkify stability policy

[aphillips]

Fixes #930

Also clarifies that [!IMPORANT] blocks are normative.

[macchiati]

Just one item

[macchiati]

One additional item. We should add the following clause for clarity.

Suggested change

	Updates to this specification may **deprecate** _functions_, _options_, and _operands_ defined in the default registry, as is standard practice in Unicode specifications. This is to indicate that the use of those _functions_, _options_, or _operands_ is discouraged. However, they are never removed, and implementations can continue to support them.

[aphillips]

We already say this (line 146), except we don't mention operands in the stability policy and, of course, it's much less explanatory. Note that the stability policy is about what we won't do.

Making this change would be normative and require WG consensus, so I'll change the tags and remove fast-track until we discuss Monday.

[macchiati]

Sounds good

[eemeli]

Eh, this is a completely separate change from this PR. It really should be handled separately, but we're going to feature-creep it into this same change once again, yes?

Something like this was previously discussed here: #883 (comment), and the spec text already includes this, which is in an [!IMPORTANT] block and therefore normative:

message-format-wg/spec/README.md

Line 142 in f3344b5

> - Future versions may deprecate (but not remove) keywords, functions, options, or option values.

[aphillips]

Eh, this is a completely separate change from this PR. It really should be handled separately, but we're going to feature-creep it into this same change once again, yes?

Under normal circumstances, I would insist on it. But I'm hoping to be done tomorrow.

[eemeli]

The current text looks good. I don't think the added deprecation item is necessary, as we already mention it later in the text. If there's something missing from that (such as explicitly allowing for deprecating operands), it ought to be modified in place.