DOC: clarify BITPOS behavior on non-existent vs empty keys

[itsMathew]

Fixes redis/redis#15987

The BITPOS documentation currently states:

Non-existent keys are treated as empty strings.

However, BITPOS distinguishes between a missing key and an existing empty string (""):

Non-existent key (o == NULL): Treated as an infinite array of 0 bits.

127.0.0.1:6379> BITPOS nokey 0
(integer) 0
127.0.0.1:6379> BITPOS nokey 1
(integer) -1

Existing empty string (""): Contains 0 allocated bytes/bits.

127.0.0.1:6379> BITPOS empty 0
(integer) -1
127.0.0.1:6379> BITPOS empty 1
(integer) -1

As per discussion in redis/redis#15987 this is the intended behaviour.
This PR updates the final note on the page to accurately describe this difference and prevent confusion.

---

Note

Low Risk
Documentation-only change to BITPOS; no runtime or API behavior changes.

Overview
Updates the BITPOS command docs to fix misleading behavior for missing vs empty keys.

The old note said non-existent keys are treated like empty strings. The new text explains that a missing key is modeled as an infinite array of 0 bits (BITPOS … 0 → 0, BITPOS … 1 → -1), while an existing key with value "" has no bits (BITPOS for 0 or 1 → -1). This matches intended server behavior (redis/redis#15987).

^{Reviewed by Cursor Bugbot for commit 21ee67c. Bugbot is set up for automated code reviews on this repo. Configure here.}

[CLAassistant]

All committers have signed the CLA.

[dwdougherty]

Thank you for this contribution, @itsMathew! Much appreciated. Approved.