docs: update the man page with --quiet

doc/python-coverage.1.txt

@@ -123,6 +123,9 @@ COMMAND REFERENCE
     \--keep
         Keep original coverage data files.
 
+    \-q, --quiet
+        Don't print messages about what is happening.
+
 **debug** `TOPIC` ...
 
     Display information about the internals of coverage.py, for diagnosing
@@ -175,6 +178,9 @@ COMMAND REFERENCE
         Number of digits after the decimal point to display for
         reported coverage percentages.
 
+    \-q, --quiet
+        Don't print messages about what is happening.
+
     \--show-contexts
         Annotate lines with the contexts that executed them.
 
@@ -217,6 +223,9 @@ COMMAND REFERENCE
     \--pretty-print
         Format the JSON for human readers.
 
+    \-q, --quiet
+        Don't print messages about what is happening.
+
     \--show-contexts
         Include information about the contexts that executed each line.
 
@@ -336,6 +345,9 @@ COMMAND REFERENCE
     \-o `OUTFILE`
         Write the XML report to `OUTFILE`. Defaults to ``coverage.xml``.
 
+    \-q, --quiet
+        Don't print messages about what is happening.
+
     \--skip-empty
         Skip files with no code.