Next to all documentation for `decompression_executor` and `interpretation_executor`, warn about limited usefulness

[jpivarski]

Especially in uproot.dask, where such use implies nested parallelism and the dangers of that.

They shouldn't be very detailed results, since there isn't room for that in a single-line argument documentation, but just give a number (1 or 2 significant figures) in the best cases and indicate which those are:

• decompression_executor for LZMA-compressed data, large TBaskets
• interpretation_executor for AsObjects interpretations that can be successfully interpreted with AwkwardForth (without a deep discussion of what that means; hopefully, web-search will help the curious)