docs: Correct return type of Search.find()

[ferdnyc]

Description of changes:

The documentation comments for Search.find() claim it can return Range or false (changed from Range or boolean in 7e7f329), but neither is accurate. firstRange (the returned value) will always be either a Range, or null when the inner anonymous function returns false.

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts