doc: possible error domains of GSubprocess

AFAIK the two possible error domains are G_SPAWN_ERROR and
G_SPAWN_EXIT_ERROR (the latter is used when calling
g_spawn_check_exit_status(), which is done by g_subprocess_wait_check()
for example).

The documentation is added to the main description of GSubprocess, not
for each individual functions.
doc: trivial fix

GTK-Doc generates a list if the "1." is at the beginning of the line.