Fix help for `--process-total-child-memory-usage` and `--process-per-child-memory-usage` (#15733)
Fix help for `--process-total-child-memory-usage` and `--process-per-child-memory-usage` (#15733)

In particular, the `--process-total-child-memory-usage` flag was setting `default_help_repr="1GiB",`, even though it did not have a `default`.

Additionally, add some clarifications around which processes are impacted, and what happens when `--process-total-child-memory-usage` is not set. 

[ci skip-rust]
[ci skip-build-wheels]
pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,benjyw/pants