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.
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.