Add missing API documentation comments and fix doc references

[google-labs-jules]

Add missing documentation comments for public constructors and fix unresolved doc reference in FlutterMediaSession to achieve 100% pub.dev API documentation score.

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PR created automatically by Jules for task 4308840683958893447 started by @wyrindev

[google-labs-jules]

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[wyrindev]

Tweak the docs a bit!

Please replace every em-dash throughout the dartdoc comments.

Also, double-check and fill in the missing documentation for these symbols:

• flutter_media_session
• flutter_media_session.MediaSessionAdapter.MediaSessionAdapter.new
• flutter_media_session_method_channel
• flutter_media_session_method_channel.MethodChannelFlutterMediaSession.MethodChannelFlutterMediaSession.new
• flutter_media_session_platform_interface

When you go through all the dartdoc, make sure it stays clear and reader-friendly — no over-explaining, no unnecessary parenthetical asides, and rephrase anything that could be simpler or more natural.

[google-labs-jules]

Tweak the docs a bit!

Please replace every em-dash throughout the dartdoc comments.

Also, double-check and fill in the missing documentation for these symbols:

• flutter_media_session
• flutter_media_session.MediaSessionAdapter.MediaSessionAdapter.new
• flutter_media_session_method_channel
• flutter_media_session_method_channel.MethodChannelFlutterMediaSession.MethodChannelFlutterMediaSession.new
• flutter_media_session_platform_interface

When you go through all the dartdoc, make sure it stays clear and reader-friendly — no over-explaining, no unnecessary parenthetical asides, and rephrase anything that could be simpler or more natural.

Thank you for the feedback! I have removed all em-dashes across the dartdoc comments, added missing library doc comments (flutter_media_session, flutter_media_session_method_channel, flutter_media_session_platform_interface), and documented constructors for MediaSessionAdapter.new and MethodChannelFlutterMediaSession.new. I also simplified and cleaned up the documentation text throughout.

[wyrindev]

Update CHANGELOG.md (actions skip)

[google-labs-jules]

Update CHANGELOG.md (actions skip)

Updated CHANGELOG.md with a 3.0.8 entry documenting the API documentation and comment improvements.