Add the dynamic_context setting to the sample coveragerc file

[Mariatta]

Is your feature request related to a problem? Please describe.
Context measurement is a very useful feature, however I found that some projects do not have this setting in their coveragerc file.
As a result, I found myself needing to re-run the coverage again all over with this setting.

I think it would be great if this is a default setting in coveragerc file.

I was looking at the "SampleFile" section of the coveragerc documentation, and the sample does not include dynamic_context setting.

Describe the solution you'd like
Adding the line
[run]
dynamic_context = test_function
as an example in https://coverage.readthedocs.io/en/7.6.12/config.html#sample-file

Describe alternatives you've considered
I wish this is a default behavior.

Additional context
I think this is one of the most useful feature of coveragepy. More people should be using it.

I can create a PR if this ticket is approved.

[nedbat]

I think most people won't want context measurement because of the time it adds and the bulk in the data. Though to be fair, I think most people don't know it even exists.