Docs: Improved and fixed statistics section. (#1120)

Details for code changes:

- Changed the header text used in the Statistics.formatted() method so that
  the client time columns are now titled "ClientTime". Previously, the title
  was just "Time".
- Adjusted the unit test cases for statistics accordingly.

Details for documentation-only changes:

- Clarified in the introduction text that there are client and
  server times, and that the server time depends on the WBEM server
  returning the WBEMServerResponseTime header field.
- Added information which WBEM servers implement the WBEMServerResponseTime
  header field.
- Clarified that the Iter methods do not show up in the statistics, but the
  WBEM operations they drive.
- Clarified that the statistics apply only to periods of time where the
  statistics on the connection are enabled.
- Added several usage examples for the statistics.
- In the Statistics and OperationStatistic classes, clarified for a number
  of methods that they are called by pywbem, to avoid confusion about their
  need to be called by the pywbem user.
- Changed the descriptions for the Statistics and OperationStatistic
  classes and their properties and methods to be more aligned with what the
  pywbem user needs to know, as opposed to what the pywbem code using them
  needs to know.
- Clarified in the descriptions of the OperationStatistic properties
  providing elapsed time values, which ones are client time vs. server time,
  and that server time is 0 when the WBEM server does not implement the
  WBEMServerResponseTime header field.
- In the description of the WBEMConnection.stats_enabled property, added a
  statement that the enablement status can also be set when creating the
  connection.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

docs/changes.rst

@@ -386,6 +386,9 @@ Enhancements
 * Docs: Improved and fixed the description of `WBEMConnection`
   operation methods. (Issue #1110).
 
+* Docs: Improved and fixed the description of the pywbem statistics
+  support. (Issue #1115).
+
 Bug fixes
 ^^^^^^^^^