ghash: Clarify that g_hash_table_add() always consumes the key
ghash: Clarify that g_hash_table_add() always consumes the key

Even if the key already exists in the table, `g_hash_table_add()` will
call the hash table’s key free func on the old key and will then replace
the old key with the newly-passed-in key. So `key` is always `(transfer
full)`.

In particular, `key` should never need to be freed by the caller if
`g_hash_table_add()` returns `FALSE`.

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

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