docs: explain benchmark summary statistics

[ColumbusLabs]

Closes #443.

Summary

• explain that Time reports the mean wall-clock duration across timed runs
• define σ as the corrected sample standard deviation and describe what its size conveys
• explain that User and System are average CPU-time components
• keep the README and manpage wording aligned

Validation

• cargo test --locked (102 passing)
• cargo fmt -- --check
• rendered manpage inspection
• mandoc lint reported no diagnostics on the added lines
• git diff --check

Windows CPU-time wording was verified from the job-object implementation; runtime verification was performed on the local platform.