Update the Known Issues with the new Pantheon Compatibility feature

[scottbuscemi]

Expected behavior

• Documentation notes that the Pantheon mu-plugin now adds a compatibility layer that applies many of these fixes
• Specific fixes that are auto-applied are noted in the doc

Actual behavior

• Documentation does not mention the mu-plugin

Suggested Resolution

• Update the docs
• [process] When new issues are reported, and this page is updated, open an issue in the mu-plugin repository to apply the fix (or the warning/error) to the mu-plugin's compatibility layer (see https://github.com/pantheon-systems/pantheon-mu-plugin/issues/52 and https://github.com/pantheon-systems/pantheon-mu-plugin/issues/53)