Remove wrong outlive requirements for Cache in docs

[zheland]

Phrase Cache must outlive DB instance which uses it. was added to docs of BlockBasedOptions::set_block_cache and Options::set_row_cache in #446.

But this requirement for Cache methods argument should no longer be valid after #497 was merged because it:

Wrap Cache and Env in an Rc and keep a copy of that in the database to ensure that they are not dropped before the database is.

I suggest to remove these outdated requirements from docs.

rust-rocksdb/src/db_options.rs

Lines 401 to 410 in b993275

	/// a block is the unit of reading from disk). Cache must outlive DB instance which uses it.
	///
	/// If set, use the specified cache for blocks.
	/// By default, rocksdb will automatically create and use an 8MB internal cache.
	pub fn set_block_cache(&mut self, cache: &Cache) {
	unsafe {
	ffi::rocksdb_block_based_options_set_block_cache(self.inner, cache.0.inner.as_ptr());
	}
	self.outlive.block_cache = Some(cache.clone());
	}

rust-rocksdb/src/db_options.rs

Lines 2823 to 2832 in b993275

	/// Sets global cache for table-level rows. Cache must outlive DB instance which uses it.
	///
	/// Default: null (disabled)
	/// Not supported in ROCKSDB_LITE mode!
	pub fn set_row_cache(&mut self, cache: &Cache) {
	unsafe {
	ffi::rocksdb_options_set_row_cache(self.inner, cache.0.inner.as_ptr());
	}
	self.outlive.row_cache = Some(cache.clone());
	}

rust-rocksdb/src/db_options.rs

Lines 50 to 51 in b993275

	#[derive(Clone)]
	pub struct Cache(pub(crate) Arc<CacheWrapper>);

rust-rocksdb/src/db_options.rs

Lines 38 to 48 in b993275

	pub(crate) struct CacheWrapper {
	pub(crate) inner: NonNull<ffi::rocksdb_cache_t>,
	}
	impl Drop for CacheWrapper {
	fn drop(&mut self) {
	unsafe {
	ffi::rocksdb_cache_destroy(self.inner.as_ptr());
	}
	}
	}