Build differences between `v2-run` and `v2-exec` not documented

**Describe the bug**
Differences between `v2-run` and `v2-exec` w.r.t. build-related behavior is not documented.

**To Reproduce**
Observe that when running:

```
$ mkdir exec-vs-run && cd exec-vs-run
$ cabal init -n
$ cabal v2-build
...
$ cabal v2-exec exec-vs-run
Hello, Haskell!
$ sed -i 's/Hello/Bonjour/g' Main.hs
$ cabal v2-exec exec-vs-run
Hello, Haskell!
$ cabal v2-run exec-vs-run
...
Bonjour, Haskell!
```

that `v2-exec` will not build or rebuild a project to incorporate any recent changes, whereas `v2-run` will.

**Expected behavior**
Cabal documentation alludes to this difference.

**System information**
- Operating system: Any? (MacOS in my case)
- `cabal` version 3.0.0.0, `ghc` version 8.8.3

Start by reviewing the existing Cabal documentation for the v2-run and v2-exec commands, then reproduce the shown edit-and-run sequence. Document the build and rebuild difference clearly, including when each command incorporates source changes, and verify the wording against both commands.

Written by the indexing model from the issue text.