Document baseline validation "strict mode"

Baseline package validation now also has a strict mode which we should probably document (similar to the other package validator documents): `EnableStrictModeForBaselineValidation`.

You want to set this to true when you want the apicompat to perform "equality" checks instead of compatibility checks. This means that api additions or changing the assembly (in a compatible way) is disallowed. The primary use-case for this is servicing in which API additions are usually forbidden.

You could also enable this so that APICompat records all compatibility differences in the suppression file (for tracking API changes). For that you would also need to set `ApiCompatGenerateSuppressionFile` to true.

_Originally posted by @ViktorHofer in https://github.com/dotnet/docs/pull/38533#discussion_r1412026574_

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[Associated WorkItem - 189677](https://dev.azure.com/msft-skilling/Content/_workitems/edit/189677)