perf python: Improve perf script -l descriptions

Add module docstrings to counting.py, tracepoint.py, and twatch.py. This
satisfies pylint's missing-module-docstring check and prepares these
existing standalone scripts in tools/perf/python/ for the later commit
in this series ("perf script: Support standalone scripts and remove
embedded scripting"), which updates 'perf script -l' to scan
tools/perf/python/ and display each script's module docstring:

```
$ perf script -l
List of available scripts:
...
  counting
    Example for counting perf events.
...
  tracepoint
    Example showing how to enable a tracepoint and access its fields.
  twatch
    Example to show how to enable a software event and track
    context switches.
...
```

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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