Spec: clarify inconsistency for [V]PADDING/DROP cell content vs. padding bytes

This ticket is split off from #26228 (moved).

Here's the relevant description from that ticket: Replying to dmr:

[...] ==== Unclear areas Here are the points that need clarification / specification:

• [...]
• spec is a bit inconsistent with PADDING cells ^^[1^^]^^[2^^] [...]

==== Inconsistency: PADDING cell payload (see bullet above)

These references highlight the inconsistency:

^^[1^^] PADDING: Payload is unused. per [3 "Cell Packet format"]. implies 0 bytes of payload, so the rest should be padded per that section ^^[2^^] The contents of a PADDING, VPADDING, or DROP cell SHOULD be chosen randomly, and MUST be ignored. per [7.2 "Link padding"]. implies the payload of a PADDING cell actually is the rest of the size of the cell, and that it SHOULD be chosen randomly

The PADDING cells were mentioned in IRC but not discussed. I think a simple change to make the spec consistent between the two sections would be this: {{{ PADDING: Payload contains random data. (See Sec 7.2) }}}

However, given the other points here, is that correct?