fixup clarify the calculation

Author: Thomas Kiley

src/ansi-c/ansi_c_internal_additions.cpp

@@ -126,8 +126,14 @@ using max_alloc_sizet = uint64_t;
 /// The maximum allocation size is determined by the number of bits that
 /// are left in the pointer of width \p pointer_width.
 ///
-/// The offset needs to be able to represent up to the allocation_size, and
-/// down to -allocation_size, hence 2^(pointer_width - object_bits - 1)
+/// The allocation size cannot exceed the number represented by the (signed)
+/// offset, otherwise it would not be possible to store a pointer into a
+/// valid bit of memory. Therefore, the max allocation size is
+/// 2^(offset_bits - 1), where the offset bits is the number of bits left in the
+/// pointer after the object bits.
+///
+/// The offset must be signed, as a pointer can point to the end of the memory
+/// block, and needs to be able to point back to the start.
 /// \param pointer_width: The width of the pointer
 /// \param object_bits : The number of bits used to represent the ID
 /// \return The size in bytes of the maximum allocation supported.
@@ -139,7 +145,7 @@ max_malloc_size(std::size_t pointer_width, std::size_t object_bits)
   PRECONDITION(object_bits >= 1);
   const auto offset_bits = pointer_width - object_bits;
   // We require the offset to be able to express upto allocation_size - 1,
-  // but also down to -allocation_size + 1, therefore the size is allowable
+  // but also down to -allocation_size, therefore the size is allowable
   // is number of bits, less the signed bit.
   const auto bits_for_positive_offset = offset_bits - 1;
   PRECONDITION(