Add note for textformat underline styles

[ashishkum-ms]

Adds a note in TextFormatUpdateEvent to mention that UAs can choose to alter the styles before dispatching textformatupdate event to combat fingerprinting.

This is not a normative change of behavior, but rather a clarification of flexibility available to UAs

Closes #8

• 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)