vm: document that setting vfs_cache_pressure to 0 isn't a good idea

At the default value of vfs_cache_pressure=100 the kernel will attempt to

reclaim dentries and inodes at a "fair" rate with respect to pagecache and

swapcache reclaim.  Decreasing vfs_cache_pressure causes the kernel to prefer

to retain dentry and inode caches.  Increasing vfs_cache_pressure beyond 100

to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will

never reclaim dentries and inodes due to memory pressure and this can easily

lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100

causes the kernel to prefer to reclaim dentries and inodes.

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