docs(core): annotate the three tracked shared functions in utils.rs

[inureyes]

Summary

docs/code-guidelines.md opens with the shared-function Used by: rule and names create_causal_mask, softcap and repeat_kv in utils.rs as the components to track. Two of the three carried no annotation, and the third carried one that was actively wrong. This adds accurate annotations to all three and records the policy for helpers with too many callers to enumerate.

Every list was derived by grep at this commit, not copied from the issue body or from memory.

What changed

• src/lib/mlxcel-core/src/utils.rs, create_causal_mask: replaced the stale list (it named Llama, Mixtral, Cohere, Phi, GLM4 and StarCoder2, none of which call it any more) with a rule-level annotation. 44 non-test callers under src/models grouped by why they materialize an explicit mask (hybrid and mixed-layer stacks, sliding-window and chunked families, VLM decoders, MLA and custom-attention decoders), the non-src/models callers, an explicit "not used by" for the mainstream dense decoders that rely on fused-SDPA implicit causality, and the grep one-liner that regenerates the exact set.
• src/lib/mlxcel-core/src/utils.rs, repeat_kv: literal list of all 14 callers (12 under src/models, plus src/vision/encoders/deepseekocr_qwen2.rs and src/audio/qwen3_omni_moe/speech_layers.rs) with the reason most decoders never call it.
• src/lib/mlxcel-core/src/utils.rs, softcap: names its single production caller, RecurrentGemma, and records that Gemma 2 and Gemma 3 route through the fused compiled_softcap / compiled_softcap_sdpa kernels instead, so a change here does not reach them.
• docs/code-guidelines.md: new "When the caller list is too long to enumerate" section holding the policy (rule plus representatives plus a "not used by" half plus the regenerating grep), with create_causal_mask as the worked example. Also notes that public items use ///, unlike the // in the older format example.

All three annotations use /// at the end of the doc block, matching the neighbouring create_causal_mask_with_left_padding in the same file.

Measured counts

The issue's table gave 46 / 12 / 1. Measured at this commit:

Function	Issue	Measured	Note
create_causal_mask	46	44 non-test (46 incl. 2 test files)	grep -rln '\bcreate_causal_mask(' src/models src/multimodal src/vision --include='*.rs'
repeat_kv	12	12 under src/models, 13 with src/vision, 14 with src/audio	grep -rln '\brepeat_kv(' src --include='*.rs'
softcap	1	1 production caller	grep -rn '\bsoftcap(' src --include='*.rs'

The issue also recorded create_causal_mask as unannotated. It did carry an annotation at HEAD, but a stale one, which is why this replaces rather than adds.

Test plan

• Diff is provably comment-only in the .rs file: every added and removed line matches ^\s*(///|//).
• cargo fmt --check clean.
• python3 scripts/ci/check_cross_repo_refs.py clean.
• Each family named in an annotation verified by per-file grep; each family in the "not used by" list verified to have zero matches.

Closes #1110