gstring: fully document semantics of @len for g_string_insert_len

The g_string_insert_len method accepts '-1' for its len parameter, as a shorthand for strlen(val). Likewise the various convenience wrappers around it also accept -1. This was not documented, leaving developers to wonder why len is a gssize, instead of gsize.

Signed-off-by: Daniel P. Berrangé berrange@redhat.com