Prompted by #3071, this clarifies that `g_test_trap_subprocess()` uses
`g_child_watch_source_new()` internally, so it will not work if any of
the preconditions for using that API are not met. In particular, if
`SIGCHLD` is ignored, things will break.

This documentation is not meant to be an API guarantee which constrains
the implementation of `g_test_trap_subprocess()` in future, just a tip
to people currently using the API.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3071