doc: Add explicit warning against mainnet use for bdk_testenv DESCRIPTORS

### Description

The published bdk_testenv crate currently exports pub const DESCRIPTORS via its utils module. These constants embed widely known test-vector extended private keys. The keys work exactly as designed. While designed strictly for regtest and test environments, there is a risk that a downstream developer might copy-paste these descriptors into a live mainnet application.

Since these private keys are entirely public, any real funds sent to their derived addresses will be immediately swept by chain-monitoring bots.