docs: document non-obvious embed behavior in custom_embed_blocks

[beeswaxpat]

Description

Adds a "Behavior that is easy to miss" section to doc/custom_embed_blocks.md covering four things the API does not make obvious, each verified against master:

• EmbedContext.node is detached for custom embeds (text_line.dart re-creates the node via CustomBlockEmbed.fromJsonString), so node.documentOffset is unusable there, while plain embeds such as images keep the attached node. Documents the correct ways to locate the embed.
• EmbedBuilder.expanded controls block vs inline rendering; expanded => false routes a single-embed line through the inline WidgetSpan path so text can share the line.
• Sizing an embed works only through the style attribute (ResolveImageFormatRule accepts only that key); WidthAttribute/HeightAttribute throw FormatException in formatText, and a try/catch turns that into a silent no-op.
• Keying editable embed widgets by a stable id so their state survives the rebuild a replaceText commit triggers.

Docs only, no code changes.

Related Issues

None.

Type of Change

• 📝 Documentation: Updates or additions to documentation.

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