Documentation: clarify that cache forgets credentials if the system restarts

Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential-cache.txt

@@ -14,10 +14,13 @@ git config credential.helper 'cache [<options>]'
 DESCRIPTION
 -----------
 
-This command caches credentials in memory for use by future Git
-programs. The stored credentials never touch the disk, and are forgotten
-after a configurable timeout.  The cache is accessible over a Unix
-domain socket, restricted to the current user by filesystem permissions.
+This command caches credentials for use by future Git programs.
+The stored credentials are kept in memory of the cache-daemon
+process (instead of written to a file) and are forgotten after a
+configurable timeout. Credentials are forgotten sooner if the
+cache-daemon dies, for example if the system restarts. The cache
+is accessible over a Unix domain socket, restricted to the current
+user by filesystem permissions.
 
 You probably don't want to invoke this command directly; it is meant to
 be used as a credential helper by other parts of Git. See