[FOWEPJF255]

What and why

Addresses #4716. The --fast manual currently describes the speed/ratio trade-off but leaves the accepted range and out-of-range behavior unclear. This documents that # must be positive, that values above the linked library's maximum are clamped, and that --fast=1 and --fast=4 are useful starting points to benchmark on the user's data.

The bounds and clamping wording follows ZSTD_minCLevel() and the --fast parser in programs/zstdcli.c. The two example levels are already used in the README benchmark table. This is complementary to #4663, which notes that --fast maps to negative library compression levels.

Checklist

• I searched open pull requests and issues: this specific bounds and selection guidance is not already being addressed.
• Documentation-only change; no code behavior changed.
• I stated below whether AI or other automated tools were used.

Validation

• Reviewed the CLI parser, ZSTD_minCLevel() implementation, and README examples against the wording.
• git diff --check passed.

Tools used

OpenAI Codex was used to inspect the source and draft this documentation. The final wording and source references were checked against the repository files.

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