Add differences for composition events and canvas element

[ashishkum-ms]

• Clarify that composition events are not fired on the editing host (they're fired on the EditContext instead).
• Clarify that when the editing host is a <canvas>, the UA can't handle selection/caret navigation/spelling and grammar checking.

This is not really a normative change of behavior, but rather clarifying behavior that the spec left unclear.

Closes #72
Closes #86.

• For browsers that are shipping the feature, implementation bugs are filed for the proposed changes (link to bug, or write "Not Implementing"):
  • WebKit (Request for Position: EditContext WebKit/standards-positions#243)
  • Chromium (https://issues.chromium.org/issues/40642681)
  • Gecko (EditContext mozilla/standards-positions#199)

[dandclark]

Thank you for working on this! I had a few suggestions about the wording.