Refactor docs for i2c2midi

[attowatt]

What does this PR do?

Updates the documentation for i2c2midi OPs:

• Added better information about the behavior of MIDI channels (using channel-specific OPs vs normal OPs)
• Added hint to use ch = 0 to make a setting for all MIDI channels
• Added allowed range of channel parameter for channel-specific OPs
• Fixed some typos

Provide links to any related discussion on lines.

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How should this be manually tested?

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Any background context you want to provide?

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I have,

• [ ] updated CHANGELOG.md and whats_new.md
• [X] updated the documentation
• [ ] updated help_mode.c (if applicable)
• [ ] run make format on each commit
• [ ] run tests