Applied Percona's Theme and markup refactory

docs/user/source/environment.rst

@@ -0,0 +1,15 @@
+
+***********
+ENVIRONMENT
+***********
+
+The environment variable \ ``PTDEBUG``\  enables verbose debugging output to STDERR.
+To enable debugging and capture all output to a file, run the tool like:
+
+.. code-block:: perl
+
+    PTDEBUG=1 pt-table-checksum ... > FILE 2>&1
+
+Be careful: debugging output is voluminous and can generate several megabytes
+of output.
+