etc/mpv.conf: update outdated use of cache options

The --cache option does not take a number anymore. (Oh boy, this is
going to break a lot of user configs?)

The cache site is now configured with those obscure-sounding --demuxer
options.

--cache-secs is not useful anymore. The default is very high, so the
obscure-sounding --demuxer options determine how much is cached.

Advertise the --cache-on-disk option a bit. I found it useful once, and
it will trick users into wearing out their SSD for no gain, or so.

etc/mpv.conf

@@ -95,18 +95,21 @@
 
 # cache settings
 #
-# Use 150MB input cache for everything, even local files.
-#cache=153600
+# Use a large seekable RAM cache even for local input.
+#cache=yes
+#
+# Use extra large RAM cache (needs cache=yes to make it useful).
+#demuxer-max-bytes=500M
+#demuxer-max-back-bytes=100M
 #
 # Disable the behavior that the player will pause if the cache goes below a
 # certain fill size.
 #cache-pause=no
 #
-# Read ahead about 5 seconds of audio and video packets.
-#demuxer-readahead-secs=5.0
-#
-# Raise readahead from demuxer-readahead-secs to this value if a cache is active.
-#cache-secs=50.0
+# Store cache payload on the hard disk instead of in RAM. (This may negatively
+# impact performance unless used for slow input such as network.)
+#cache-dir=~/.cache/
+#cache-on-disk=yes
 
 # Display English subtitles if available.
 #slang=en