Document "Compare active file with clipboard" functionality

[ntrogh]

Introduced in VS Code 1.19, you can compare the active file with the contents of the clipboard.

Command: File: Compare Active File with Clipboard (workbench.files.action.compareWithClipboard) Keybinding: Ctrl+k c

Docs entries:

• User Guide/Basic Editing - new section
• Source Control/Overview - update viewing diffs section
• Get Started/Tips and Tricks - update diffs section