docs: fix stack comments in counter.masm

[Mustdzyl]

counter.masm is the first contract most people read, and its stack comments don't match what the procedures actually leave on the stack.

Per the protocol docstrings:

• active_account::get_item — Inputs: [slot_id_suffix, slot_id_prefix] → Outputs: [VALUE], a word.
• native_account::set_item — Inputs: [slot_id_suffix, slot_id_prefix, VALUE] → Outputs: [OLD_VALUE], also a word.

And sys::truncate_stack only removes elements below the top 16 ("the top 16 elements of the stack remain unchanged"), so it doesn't clear OLD_VALUE either.

That makes two comments in increment_count wrong rather than merely abbreviated: it is documented as Outputs: [] and # => [], but it actually leaves the previous count on the stack. get_count is documented as returning [count] where get_item returns a full word.

The other account modules in this repo already get this right — count_reader.masm documents # => [OLD_VALUE, pad(12)] and mapping_example_contract.masm documents # => [OLD_VALUE] / # => [VALUE] — so this just brings counter.masm in line with its siblings.

Comments only; no behavioural change.

One thing I deliberately left alone: increment_count returning OLD_COUNT looks incidental rather than intended. If it should return nothing, a dropw after set_item would do it — happy to add that instead if you'd prefer the code change over the comment change.