settings: document code on when exactly the pubkey is regenerated

The pubkey should not always be regenerated when a privatekey is
pasted. The reason is that on paste event is fired before the
privateKeyInput value is updated.

Added code comments on when the condition passes and should be save
to re-generate the pubkey.

src/main.js

@@ -613,10 +613,10 @@ privateKeyInput.addEventListener('paste', (event) => {
   if (pubKeyInput.value || !event.clipboardData) {
     return;
   }
-  if (privateKeyInput.value === '' || (
-    privateKeyInput.selectionStart === 0
+  if (privateKeyInput.value === '' || ( // either privatekey field is empty
+    privateKeyInput.selectionStart === 0 // or the whole text is selected and replaced with the clipboard
     && privateKeyInput.selectionEnd === privateKeyInput.value.length
-  )) {
+  )) { // only generate the pubkey if no data other than the text from clipboard will be used
     try {
       pubKeyInput.value = getPublicKey(event.clipboardData.getData('text'));
     } catch(err) {} // settings form will call validKeys on input and display the error