Fix all Sphinx build warnings/errors (19 -> 0)

[qiyanjun]

Summary

Built the docs locally with sphinx -b html . _build/html -W --keep-going (warnings-as-errors, don't stop at the first one) and fixed everything it surfaced — 19 warnings/errors down to a clean build succeeded.

What was found and fixed

• docs/api/transformations.rst: a broken toctree block referencing textattack.transformations.{sentence_transformations,word_insertions,word_merges,word_swaps} as if this file lived in docs/apidoc/ — but it's in docs/api/, so none of the four targets resolved (toctree contains reference to nonexisting document). Removed it; the same content is already reachable via docs/apidoc/textattack.transformations.rst, linked from index.rst.
• textattack/attack_args.py: two .. note:: blocks nested directly under a docstring paragraph with no blank line before them (invalid RST nesting → Unexpected indentation). Also fixed a malformed :obj: `int` role (stray space breaks the role syntax) — turned out to be the actual cause of a separate ambiguous cross-reference warning (more than one target found for cross-reference 'num_examples_offset', matching both AttackArgs and ModelEvalArgs) once fixed.
• textattack/constraints/.../learning_to_write.py: a docstring paragraph had one extra leading space versus its sibling paragraphs, so RST parsed it as an indented block quote.
• textattack/transformations/.../back_transcription.py: a BibTeX citation block wasn't marked as a literal block, so RST tried (and failed) to parse its internal indentation as nested block-quotes/definition-lists. Marked it literal with ::, matching the Example:: block already used above it in the same docstring.
• Five files ("Title underline too short"): extended each RST title underline to match its title's actual length. One of these (named_entity_recognition_goal_function_result.py) also had a real content bug: the module docstring title was misspelled (...RecognitionoalFunctionResult, missing the "G" in "Goal") and its one-line description was copy-pasted from logit_sum's file ("logit sum goal function Result") instead of describing this class.

Why this matters now

None of these had actually broken CI's docs build before, because .readthedocs.yaml's build config itself was broken first (see #836) — RTD's build was failing at the config/install step, before ever reaching Sphinx. Once #836 lands and RTD can actually run Sphinx, these are exactly the warnings that build would hit.

Verification

• Clean local Sphinx build: build succeeded with zero warnings/errors (confirmed via -W --keep-going, both incrementally after each fix and as a final full clean rebuild)
• All edited modules still import correctly (docstring edits don't affect runtime, but confirmed no syntax breakage)
• black --check / flake8 clean on all changed files

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