[DOCS]Merge strategies not clearly documented

[hramrach]

Description
Merge strategies not clearly documented.

Suggested Fix
Specifics on what is and isn't merged and when

Type of documentation
Salt documentation

Location or format of documentation
https://docs.saltproject.io/en/latest/ref/configuration/master.html#pillar-source-merging-strategy

Additional context
With the default smart strategy I observe that some includes are merged while others not, with no obvious difference between the cases.

The documentation is missing several details:

• what strategy is selected when by the 'smart' strategy
  • is it possible to tell after the fact which was selected and why
• in the 'recursive' strategy the example only shows data that is merged, and no data that is not merged. Is it possible that some data is not merged, and what kind?

[dwoz]

@hramrach would you be willing to create a PR for this change?

[hramrach]

My problem is that I do not understand what the merge strategies do. With that I cannot write the documentation for them.