docs: make process_kill signum arg optional

[Bilal2453]

In Libuv, uv_process_kill takes a process and a signum, where the signum is required. In Luv although, in luv_parse_signal, if the input is not an integer and not a string, the default value of SIGTERM is used.

Effectively making signum an optional argument with the default string value sigterm.

[squeek502]

The relevant code:

luv/src/process.c

Lines 302 to 310 in 9e3c232

	static int luv_parse_signal(lua_State* L, int slot) {
	if (lua_isnumber(L, slot)) {
	return lua_tonumber(L, slot);
	}
	if (lua_isstring(L, slot)) {
	return luv_sig_string_to_num(lua_tostring(L, slot));
	}
	return SIGTERM;
	}

[Bilal2453]

Seems like I forgot uv.kill in this PR.