doc: improve doc of g_input_stream_read()

I recently needed to nul-terminate the returned buffer, and I wasn't
sure if g_input_stream_read() does that or not. I've checked
glocalfileinputstream.c, which calls read(2) which doesn't nul-terminate
the buffer. So I assume it's the same behavior for all GInputStream
subclasses.

https://bugzilla.gnome.org/show_bug.cgi?id=732704
doc: small improvement and fixes

- Add an example to g_strsplit(), like it is done for g_strsplit_set().

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

https://bugzilla.gnome.org/show_bug.cgi?id=732704
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.

https://bugzilla.gnome.org/show_bug.cgi?id=732704