docs: fix two docstrings that reference names the code does not have (#14733)

* docs: fix two docstrings that reference names the code does not have

- transformer_ltx2.py:1449 says `peturbation_mask`; the parameter is
  `perturbation_mask`, spelled correctly at :253, :273, :314, :316, :623.
- pipeline_stable_diffusion_diffedit.py:885,:888 point at `source_image`,
  but generate_mask's image argument is `image`.

Fixes #14732

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

* docs: fix three renamed parameters left stale in docstrings

Each documents a name the signature no longer has:

- optimization.py:161      num_periods        -> num_cycles
- autoencoder_kl_ltx2.py:43 dim               -> channel_dim
- z_image/before_denoise.py:100 vae_scale_factor -> vae_scale_factor_spatial

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>