docs: simplify CLI documentation language

[ryanfowler]

Summary

• revise CLI documentation to use shorter, active sentences and direct instructions
• improve terminology and structure across the README and user guides
• clarify that automatic uncompressed SSE retries apply only to GET and HEAD requests

Validation

• prettier -w README.md docs/**/*.md
• local Markdown link check
• git diff --check

Cargo tests were not run because these changes affect documentation only.