Update documentation/comment for GH-4860

Fix folding for the new helper method.

Clarify comment in UPGRADING:
The performance on associative arrays would also improve,
as long as no offsets were unset (no gaps).
Packed arrays can have gaps.

Closes GH-4873.
[ci skip]

Author: TysonAndre

UPGRADING

@@ -371,6 +371,6 @@ PHP 8.0 UPGRADE NOTES
 14. Performance Improvements
 ========================================
 
-- array_slice() on a packed array will no longer scan the whole array to find
+- array_slice() on an array without gaps will no longer scan the whole array to find
   the start offset. This may significantly reduce the runtime of the function
   with large offsets and small lengths.

ext/standard/array.c

@@ -3480,6 +3480,7 @@ static inline Bucket* find_bucket_at_offset(HashTable* ht, zend_long offset)
 	/* Return a pointer to the end of the bucket array. */
 	return ht->arData + ht->nNumUsed;
 }
+/* }}} */
 
 /* {{{ proto array array_slice(array input, int offset [, int length [, bool preserve_keys]])
    Returns elements specified by offset and length */