crosvm_cli: improve documentation for exit codes.
crosvm_cli: improve documentation for exit codes.

Some of the bit processing is a little hard to follow, so we've improved
the doc comment to further clarify the bit positions & meanings.

BUG=b:258279354
TEST=n/a

Change-Id: Ic103ff1a1377be6ddf2e5492d750733b80bddb6f
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4015004
Reviewed-by: Vikram Auradkar <50abc1d85bedfd527fc7dee23f4819d1f0d946ec@google.com>
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>
Reviewed-by: Richard Zhang <666ac13a3069b6c13f19fc70113e1242ca7ab9af@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm