sodium: fix return-value prose for 4 functions that throw SodiumException instead of returning false

[lacatoire]

The return values section of these four functions states they return false on failure. In php-src they never return false: every error path throws a SodiumException (zend_throw_exception / zend_argument_error followed by RETURN_THROWS()), and on success the function returns the raw binary string.

Proof in ext/sodium/libsodium.c (pinned at 1a7f3c0), none of the four bodies contains a RETURN_FALSE:

• sodium_crypto_aead_chacha20poly1305_encrypt: L2123-L2177, throws at L2143
• sodium_crypto_aead_chacha20poly1305_ietf_encrypt: L2236-L2294, throws at L2256
• sodium_crypto_aead_xchacha20poly1305_ietf_encrypt: L2359-L2413, throws at L2379
• sodium_crypto_stream_xchacha20_xor_ic: L1364-L1405, throws at L1383

[jordikroon]

It's good IDE's have autocomplete these days. Anyway back on topic. The stubs don't allow the function to return false anyway.