Add lack of support for auto-detection from stdin to the docs

[mikemc]

Running fastp from STDIN, whether interleaved paired data with --detect_adapter_for_pe, or on single-end data, runs with no error but outputs messages that auto-detection is disabled (example below). It would be helpful to note this behavior in the README and doc string.

cat test_interleaved.fq | fastp \
  --stdin \
  --interleaved_in \
  --detect_adapter_for_pe \
  --json report_stream.json \
  --html report_stream.html
Adapter auto-detection is disabled for STDIN mode
Adapter auto-detection is disabled for STDIN mode
Read1 before filtering:
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