docs(user-manual): B-04 — pip install row warns Python API only

Phase 4.5 Phase G doc-sweep follow-up. Phase F R2 (PW f20e2f6) updated
the PyPI long-description so users browsing pypi.org/project/sparrow-engine
see the API-only warning, but docs/user-manual.md install-paths table
(line 256) still said only 'Notebook + script users' with no warning
that the pip install path ships zero console scripts (spe / spe-gpu /
sparrow-engine-server).

Add parenthetical to the 'Best for' cell pointing at the brew /
installer / tarball rows above for the CLI binary.

Closes the user-manual half of B-04 (PyPI half was closed by f20e2f6).

Refs:
  docs/review/phase4.5-cleanup-audit-fix-f/round_01/rubber_duck_review.md § D2 (HIGH B-04)
  docs/review/phase4.5-cleanup-audit-fix-f/round_02/fixer_plan.md § F-R2-2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zhmiao

docs/user-manual.md

@@ -253,7 +253,7 @@ Under the stdin-pipe form the wrapper detects that `$0` is the shell name and sk
 | **Homebrew tap** | End users on macOS arm64 + brew-Linux x86_64; zero setup | `brew tap microsoft/sparrow-engine && brew install sparrow-engine` (CPU) or `brew install sparrow-engine-gpu` (Linux + NVIDIA only — auto-discovers cuDNN / nvJPEG; see §2.5) |
 | Clean-room from-source build | Developers; reproducibility | `cd sparrow-engine && ./scripts/build_all_flavors.sh` (workspace root) |
 | GitHub Releases binary | End users; production | `bash installer/sparrow-engine-install.sh --cli` |
-| pip install Python wheel | Notebook + script users | `pip install sparrow-engine` (CPU) or `pip install sparrow-engine-gpu` |
+| pip install Python wheel | Notebook + script users (Python API only — no `spe` CLI binary; use the Homebrew, installer, or tarball rows above for the CLI) | `pip install sparrow-engine` (CPU) or `pip install sparrow-engine-gpu` |
 | Docker image pull | Server deployments | `docker pull sparrow-engine:cpu` (or `:gpu`) |
 
 **Cite**: `docs/install.md § Per-consumer install paths` (lines 163-220); `sparrow-engine/scripts/build_all_flavors.sh`; `installer/homebrew/{sparrow-engine,sparrow-engine-gpu}.rb` + `installer/homebrew/README.md` (Homebrew tap source-of-truth).