doc: various improvements

- g_subprocess_launcher_spawn() and spawnv(): there is no other way
  AFAIK to create a GSubprocess from a launcher. So these
  functions are not "convenience helper".

- annotate optional arguments for g_shell_parse_argv().

- other trivial fix

https://bugzilla.gnome.org/show_bug.cgi?id=732357
doc: improve doc of g_file_equal()

A critical message is printed when a parameter of g_file_equal() is not
a GFile. When we read the documentation before this commit, we can think
that passing NULL or another type than GFile is allowed, but it is not
the case.

Another option is to allow NULL parameters. But for consistency with
e.g. g_str_equal(), it's probably better to keep the code as is.

https://bugzilla.gnome.org/show_bug.cgi?id=732357