fix: Fact-check of streams, server events, and backward compatibility

[Swiftaxe]

Summary

Fact-checked, removed noise, and added frontmatter to 3 pages.

All 3 files:

• Added missing description frontmatter

15-streams:

• Removed "Note that" weak lead-in from tip
• "utilizing the authentication data" → "using"
• Trimmed verbose auth revocation sentence
• "It's often handy to associate a state" → direct imperative sentence
• Fixed prose naming connectWebSocket when the code uses openStreamingConnection (factual fix)
• Removed trailing blank line inside code block

16-server-events:

• "the message published on the channel" → "the message is published" (grammar fix)
• Removed "Receiving messages is just as simple as sending them!" (informal + exclamation mark)
• "processes incoming requests" → "processes incoming messages"
• "you simply cancel" → "cancel"
• Removed "In the above example, the stream is first created and then canceled." (redundant narration)
• "regardless if" → "regardless of whether"

17-backward-compatibility:

• "Additionally all the other" → "Additionally, all the other" (missing comma)