@debug the next REPL line before evaling it

Add a `@debug` expression within DocTests.eval_repl to show which line is
about to be evaluated.
This was requested in
#2013

src/DocTests.jl

@@ -242,6 +242,8 @@ end
 function eval_repl(block, sandbox, meta::Dict, doc::Documenter.Document, page)
     for (input, output) in repl_splitter(block.code)
         result = Result(block, input, output, meta[:CurrentFile])
+        src_lines = Documenter.find_block_in_file(result.block.code, result.file)
+        @debug "Evaluating doctest REPL line from $(Documenter.locrepr(result.file, src_lines))" input expected_output = output
         for (ex, str) in Documenter.parseblock(input, doc, page; keywords = false, raise=false)
             # Input containing a semi-colon gets suppressed in the final output.
             result.hide = REPL.ends_with_semicolon(str)