docs(output): explain mean and sigma

[cyncui]

why

hyperfine labels mean and σ in its results, but the docs do not explain that σ describes variation between runs rather than uncertainty in the mean. closes #443.

what changed

• explain the average, corrected sample standard deviation, and single-run output in the readme and man page.

scope

• documentation only. measurement and output behavior stay the same.

blast radius

• this changes guidance for readers and has no runtime effect.

verification

• cargo +stable test --locked --quiet passed all 132 tests.
• cargo +stable fmt --all --check and clippy with -D warnings passed.
• the built cli showed mean ± σ for three runs and value for one run. mandoc rendered the new section.