MANPAGE: add -xchacha

Documentation/MANPAGE.md

@@ -100,9 +100,11 @@ Available options for `-init` are listed below. Usually, you don't need any.
 Defaults are fine.
 
 #### -aessiv
-Use the AES-SIV encryption mode. This is slower than GCM but is
+Use the AES-SIV encryption mode. This is slower than AES-GCM but is
 secure with deterministic nonces as used in "-reverse" mode.
 
+Run `gocryptfs -speed` to find out if and how much slower.
+
 #### -deterministic-names
 Disable file name randomisation and creation of `gocryptfs.diriv` files.
 This can prevent sync conflicts conflicts when synchronising files, but
@@ -142,6 +144,12 @@ Setting this to a lower
 value speeds up mounting and reduces its memory needs, but makes
 the password susceptible to brute-force attacks. The default is 16.
 
+#### -xchacha
+Use XChaCha20-Poly1305 file content encryption. This should be much faster
+than AES-GCM on CPUs that lack AES acceleration.
+
+Run `gocryptfs -speed` to find out if and how much faster.
+
 MOUNT OPTIONS
 =============