gstrfuncs: Clarify that g_ascii_string_to_unsigned() rejects signs
gstrfuncs: Clarify that g_ascii_string_to_unsigned() rejects signs

Unlike g_ascii_strtoull(), g_ascii_string_to_unsigned() does not permit
leading signs (`+` or `-`). Document that.

It’s already in the unit tests.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib