As I mentioned, this isn't Google Docs adding full markdown support. Text is converted to Docs' native formatting immediately, meaning there's no way to edit the text in markdown after that.

You can use find and replace in Google Docs to find a word in a document, to replace a single entry, or to replace every instance of a word or phrase.

To find and replace text in Google Docs on PC or Android phone easily, you can follow the instructions laid down in this tutorial.