v4.0.x and v4.1.x: document that --cpu-set doesn't work

[jsquyres]

Per #9298 and #9299, it was discovered that --cpu-set does not behave the same way that it did in Open MPI 3.1.x. Instead, it behaves like --cpu-list.

The 2 linked PRs nominally fix the --cpu-set behavior to be correct, but were ultimately rejected for two reasons:

1. The code is very complex and high risk. It feels like every time we have touched this code in the past, we have broken it.
2. We'd be changing behavior in the middle of two release series. This may be small behavior that may not affect many people, but it is a change in behavior from "something that results in a successful code run" to "some other behavior that also results in a successful code run." It feels dangerous to do that.

The compromise that we came up with is to keep the --cpu-set operational (because disabling it may change user scripts to suddenly stop working) but document that --cpu-set in 4.x is accidentally doing exactly the same thing as --cpu-list. There are probably 3 places that need to be updated:

1. README.md
2. mpirun(1) man page
3. The output from mpirun --help
4. ...? (maybe others?)

The fix for restoring --cpu-set behavior will be included in v5.0.0.

@gpaulsen volunteered to document the issue. Please verify with @awlauria that my description above is correct, and update all documentation in v4.0.x and v4.1.x accordingly.

[gpaulsen]

Sorry I haven't got this PRed. I am actively working it in between other 🔥s.

[gpaulsen]

Sorry i have to un-volunteer for this.