Add 'coverage debug' to help

coverage/cmdline.py

@@ -349,13 +349,13 @@ def get_prog_name(self):
         "debug", GLOBAL_ARGS,
         usage="<topic>",
         description=(
-            "Display information on the internals of coverage.py, "
+            "Display information about the internals of coverage.py, "
             "for diagnosing problems. "
             "Topics are: "
                 "'data' to show a summary of the collected data; "
                 "'sys' to show installation information; "
                 "'config' to show the configuration; "
-                "or 'premain' to show what is calling coverage."
+                "'premain' to show what is calling coverage."
         ),
     ),
 
@@ -831,6 +831,7 @@ def unglob_args(args):
         Commands:
             annotate    Annotate source files with execution information.
             combine     Combine a number of data files.
+            debug       Display information about the internals of coverage.py
             erase       Erase previously collected coverage data.
             help        Get help on using coverage.py.
             html        Create an HTML report.

doc/python-coverage.1.txt

@@ -43,6 +43,9 @@ COMMAND OVERVIEW
 |command| **combine**
     Combine a number of data files.
 
+|command| **debug**
+    Display information about the internals of coverage.py.
+
 |command| **erase**
     Erase previously collected coverage data.
 
@@ -116,6 +119,18 @@ COMMAND REFERENCE
         Append coverage data to .coverage, otherwise it starts clean each
         time.
 
+**debug** `TOPIC` ...
+
+    Display information about the internals of coverage.py, for diagnosing
+    problems.
+
+    Topics are:
+
+        ``data`` to show a summary of the collected data;
+        ``sys`` to show installation information;
+        ``config`` to show the configuration;
+        ``premain`` to show what is calling coverage.
+
 **erase**
 
     Erase previously collected coverage data.