Clarify the correlation output layout

[sylvesterkaczmarek]

Summary

• distinguish the srcALen + srcBLen - 1 correlation values from the larger zero-padded destination buffer
• document whether the padding appears before or after the correlation values for unequal input lengths
• retain the existing destination allocation and zero-initialisation requirements

Context

The correlation implementations intentionally use a destination length of 2 * max(srcALen, srcBLen) - 1. The existing group documentation describes that entire buffer as the correlation result, but does not show where the actual correlation values are located when the input lengths differ.

This change documents the existing layout without changing the implementation or API.

Fixes #120.

Validation

• git diff --check
• compiled the scalar arm_correlate_f32.c path with GCC 13.3.0 and -Wall -Wextra -Werror, using minimal test-only stubs for the external CMSIS Core headers
• ran focused C assertions for both unequal-length orders and confirmed the documented leading and trailing zero padding

Documentation-only change. The repository test suite was not run.