Improve doc comments on Search IndexingParametersConfiguration

In `IndexingParametersConfiguration`:

* `IndexedFileNameExtensions` & `ExcludedFileNameExtensions`: add details on what happens if both are set.
* Nullable bool properties such as `FailOnUnsupportedContentType` should specify what a null value signifies.
* `DelimitedTextDelimiter`: add information about the default value.
* `DocumentRoot`: specify what kind of path syntax should be used here.
* `PdfTextRotationAlgorithm`: "Determines algorithm for text extraction" should be "Determines the algorithm used for text extraction"

Locate the definition of IndexingParametersConfiguration and inspect the doc comments for IndexedFileNameExtensions, ExcludedFileNameExtensions, FailOnUnsupportedContentType, DelimitedTextDelimiter, DocumentRoot, and PdfTextRotationAlgorithm. Update the comments to cover the requested behavior, null meaning, default, path syntax, and wording, then verify the resulting API specification documentation.

Written by the indexing model from the issue text.