docs: leave a note in status.json pointing to the right place to look

In nedbat#1730 and
nedbat#1732, it's clear that people
might poke around and find apparently useful information in the
htmlcov/status.json file.  Add a note to try to get them to the place
they want to be.

coverage/html.py

@@ -506,6 +506,11 @@ class IncrementalChecker:
 
     STATUS_FILE = "status.json"
     STATUS_FORMAT = 2
+    NOTE = (
+        "This file is an internal implementation detail to speed up HTML report"
+        + " generation. Its format can change at any time. You might be looking"
+        + " for the JSON report: https://coverage.rtfd.io/cmd.html#cmd-json"
+    )
 
     #  The data looks like:
     #
@@ -578,6 +583,7 @@ def write(self) -> None:
             files[filename] = fileinfo
 
         status = {
+            "note": self.NOTE,
             "format": self.STATUS_FORMAT,
             "version": coverage.__version__,
             "globals": self.globals,