Improve comment in page cache (#5220)

It was easy to interpret comment in the page cache initialization code
to be about justifying why we leak here at all, not just why this
specific type of leaking is done (which the comment was actually meant
to describe).

See
https://github.com/neondatabase/neon/pull/5125#discussion_r1308445993

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

pageserver/src/page_cache.rs

@@ -799,8 +799,9 @@ impl PageCache {
     fn new(num_pages: usize) -> Self {
         assert!(num_pages > 0, "page cache size must be > 0");
 
-        // We use Box::leak here and into_boxed_slice to avoid leaking uninitialized
-        // memory that Vec's might contain.
+        // We could use Vec::leak here, but that potentially also leaks
+        // uninitialized reserved capacity. With into_boxed_slice and Box::leak
+        // this is avoided.
         let page_buffer = Box::leak(vec![0u8; num_pages * PAGE_SZ].into_boxed_slice());
 
         let size_metrics = &crate::metrics::PAGE_CACHE_SIZE;