Merge bitcoin/bitcoin#28673: docs: Add reference to total.coverage report

2ce7e31 docs: Add reference to total.coverage report (Greg Sanders)

Pull request description:

  I couldn't get fuzz coverage to work, so couldn't verify what it would look like if made.

ACKs for top commit:
  darosior:
    ACK 2ce7e31
  dergoegge:
    ACK 2ce7e31

Tree-SHA512: c773ef28f7cc630cb094740805c0d235255e7b0559ac7fed4e8952ade4005335f3787bdd2f8786979e341e914b41c4f3eef02bb4773b15f92d618f1040f53c54

Author: fanquake

doc/developer-notes.md

@@ -488,7 +488,9 @@ To enable LCOV report generation during test runs:
 make
 make cov
 
-# A coverage report will now be accessible at `./test_bitcoin.coverage/index.html`.
+# A coverage report will now be accessible at `./test_bitcoin.coverage/index.html`,
+# which covers unit tests, and `./total.coverage/index.html`, which covers
+# unit and functional tests.
 ```
 
 ### Performance profiling with perf