Add note about isTerminated

[mifi]

closes #1177

[ehmicky]

@mifi Thanks for this PR.

However, this additional explanation might add more confusion than clarity in this specific case.

If a process handles a signal, it removes the OS's default behavior, which for most signals (like SIGTERM) is to terminate the process. Therefore, a better way to put it would be:

"When a subprocess was terminated by a signal, error.isTerminated is true. Please note that if a subprocess handles a non-forceful termination signal (like SIGTERM), it is not terminated by default anymore."

However that last sentence is rather unrelated to Execa. Execa does not know whether the process handled a termination signal or not. It just knows whether the process was terminated by a signal or not.

That sentence is also not always true. While this is how Unix behaves at the OS level, some specific runtimes or tools might behave differently. For example, a test runner could let users handle SIGTERM, but add its own event handler to still terminate the process with a signal afterwards.

In the end, the only thing that is knowable from a OS standpoint is whether the process was ultimately terminated by a signal or not, which is what error.isTerminated is representing.