docs/vfs: unclear description of --vfs-cache-max-age

Hi. First of all, kudos to you for the VFS cache - it's really brilliant. I have one request though.

For a little background, as of now I'm using the following settings:

--vfs-cache-mode full \
  --vfs-cache-max-size 500G \
  --vfs-cache-max-age 5000h \
  --vfs-cache-poll-interval 5m \
  --vfs-read-ahead 2G \
  --bwlimit-file 32M \
  --vfs-read-chunk-size=64M \
  --vfs-read-chunk-size-limit=2048M \

I allocate 500 GB to the VFS cache. Right now, because of how vfs-cache-max-age is set, when I run out of space - the oldest files will be removed to free it up. It's already a nice solution, however I'd like to have a bit more flexibility:

1 - Delete cache files that haven't be accessed is X days 2 - When you run out of space => delete the least opened files

The first option would be nice because oldest != less relevant. There may be very old files that still get accessed and shouldn't be removed.

What do you guys think? I think it would be a nice thing to have.