GO-7383 Document the cross-space object link in the block text syntax

[requilence]

What

The served block-text syntax (object.schema.json, five text-block slots) listed the same-space object link only. It now also names the platform's two-parameter deep link:

[text](anytype://object?objectId=<id>&spaceId=<space_id>)   links to an object in another space

INLINE_MARKUP.md's row for that form now calls it the cross-space object link, keeps the rule that a second parameter makes it not-an-object-link here (the codec keeps it as an ordinary link, param verbatim), and notes that the API expands a short space id in it to the full one on write.

Why

Round-six eval: at call 48 a served schema handed an agent the block-text syntax and it had no way to reference an object in another space. The link form already exists (core/block/export/writer.go emits it); only the served text was silent about it.

Notes

• No parser change. The codec still parses only the exact one-parameter form into an Object mark.
• The description stays under the 800-character slot cap (TestPublishedProse), and the guide row keeps the sentence TestInlineMarkupGuideMatchesTheParser pins.
• The heart side (anytype-heart go-7383-typed-hints-followup, f3789ca70) rewrites a short spaceId in such a link to the full id on every path that turns caller text into marks, and bumps to this commit.