Docs: correct what transform() does to indexes, views and triggers

[dchaudhari7177]

Closes #849.

The line under Custom transformations with .transform_sql() says .transform() "does not automatically upgrade indexes, views or triggers". All three parts are wrong — and the trigger one is wrong in the dangerous direction.

Measured on current main, across all three kinds of transform:

rename a column   triggers ['trg'] -> []   views kept   indexes kept
change a type     triggers ['trg'] -> []   views kept   indexes kept
drop a column     triggers ['trg'] -> []   views kept   indexes kept

• Indexes are handled. transform_sql() captures each one before the old table is dropped and reissues its CREATE INDEX, rewriting a renamed column: CREATE INDEX "idx" ON "t" ("new_name").
• Views are kept — and were already documented accurately under Tables referenced by views, so the old sentence also contradicted a section four paragraphs above it.
• Triggers are dropped, not "not upgraded". SQLite deletes a table's triggers with the table, and transform_sql() never captures them.

That last one is why I'd treat this as more than a wording tidy. A reader told a trigger was not upgraded reasonably expects it to still exist, possibly stale. It is gone — silently, on every transform(), including one called with no arguments to reformat the schema.

What replaces it

A short Indexes and triggers section covering what actually happens, including two details I had to read transform_sql() to get right:

• an index with no stored CREATE INDEX (one implied by a UNIQUE constraint) is not reissued, because the constraint is reproduced in the new CREATE TABLE instead — verified, UNIQUE survives;
• any other index without stored SQL raises TransformError telling you to drop and recreate it yourself.

The trigger part carries a recipe using table.triggers_dict, with the caveat that a captured trigger body still names the old columns.

Verification

Everything asserted above was run against main before being written, including the recipe:

triggers_dict works:        ['trg']
after transform, triggers:  []
after reissue,   triggers:  ['trg']
index recreated on       :  CREATE INDEX "idx" ON "t" ("new_name")
UNIQUE kept in new schema:  True

sphinx-build -b html -W --keep-going docs    clean

The issue also suggested the views wording needed changing. I left the Tables referenced by views section alone — it is already precise, including the no such column case — and only removed the stale sentence that contradicted it.

Happy to split the trigger behaviour into its own issue if you'd rather transform() preserved them instead; this PR only documents what it does today.

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📚 Documentation preview 📚: https://sqlite-utils--854.org.readthedocs.build/en/854/