docs: clarify byte units for size-based ulimits (memlock)

[locker95]

- What I did

Documented the units for size-based --ulimit values.

The values of --ulimit options are passed to the setrlimit(2) syscall as-is, so size-based limits such as memlock, data, and fsize are expressed in bytes. The ulimit shell builtin, however, reports several of these limits in kilobytes, so --ulimit memlock=4096 produces a limit that ulimit -l reports as 4. That's counter-intuitive for users expecting the same units as the shell, and it wasn't documented. Values must also be plain integers; unit suffixes such as k, m, or g aren't supported.

This PR only clarifies the documentation and doesn't change any behavior. Accepting human-friendly suffixes (for example --ulimit memlock=64m) could be a follow-up enhancement if maintainers think it's worthwhile, but that changes option parsing and deserves its own discussion.

Fixes #5799

- How I did it

• Extended the --ulimit section in docs/reference/commandline/container_run.md: documented that size-based limits are in bytes (following setrlimit(2) semantics) and that values must be plain integers without unit suffixes, and added a note with a memlock example showing the byte value (67108864 = 64 MiB) next to the corresponding ulimit -l output (65536, in kilobytes).
• Expanded the terse --ulimit entry in man/docker-run.1.md with the same information.

- How to verify it

Documentation-only change; review the rendered Markdown. The documented behavior can be verified with:

$ docker run --ulimit memlock=67108864:67108864 --rm debian sh -c "ulimit -l"
65536

- Human readable description for the release notes