docs: document DELETE and UPDATE for SQL users and table provider authors

[michaelsembwever]

Which issue does this PR close relate to?

• Documentation follow-up for feat: Add DELETE/UPDATE hooks to TableProvider trait and to MemTable implementation #19142, which added the TableProvider row-level DML hooks and closed Support delete_from and update in TableProvider #16959. That PR shipped no documentation.
• Related to UPDATE ...FROM bug #19950.

Rationale for this change

Since 52.0.0, DataFusion runs DELETE and UPDATE against a table whose provider implements TableProvider::delete_from() or TableProvider::update(), and the built-in in-memory table implements both. No page in the documentation says so.

A SQL user therefore cannot learn which tables accept the two statements, what a statement returns, or which forms fail. A provider author cannot learn what the planner passes to each hook, or what the hook must return.

Two current behaviours are surprising enough to warn about in the same pass:

• A DELETE or an UPDATE whose WHERE clause holds an IN or an EXISTS subquery applies to all rows of the table. The optimizer rewrites the subquery into a LeftSemi Join, so extract_dml_filters() finds no predicate on the target table, and the provider reads the
  empty filter list as "no WHERE clause".

  > create table s1 as values (1), (2), (3);
  > create table s2 as values (2);
  > delete from s1 where column1 in (select column1 from s2);
  -- count 3; s1 is now empty

• EXPLAIN DELETE and EXPLAIN UPDATE execute the statement on an in-memory table. MemTable changes the rows inside the hook, and the physical planner calls the hook while it builds the plan.

Both behaviours need code fixes, which this PR does not attempt. Until then a reader needs the warning.

What changes are included in this PR?

docs/source/user-guide/sql/dml.md:

• A DELETE section and an UPDATE section: syntax, the count result, three-valued logic, and examples.
• A "Table support for DELETE and UPDATE" section: which table kinds support the statements, and the exact error text for a table that does not.
• A "Limitations" section: the two warnings above, the ignored LIMIT on DELETE, and UPDATE ... FROM.

docs/source/library-user-guide/custom-table-providers.md:

• A "Row-Level DML: DELETE and UPDATE" section: what the planner passes to each hook (split AND conjunctions, stripped table qualifiers, target-table predicates only), the single-row count return contract, the two semantic rules a provider must follow, a compiling example,
  the clauses a hook never receives, and when the work happens.

No code changes.

Are these changes tested?

Yes.

• cargo test --doc -p datafusion library_user_guide_custom_table_providers passes. The new example is a compiled doctest, not an ignore block.
• ./ci/scripts/doc_prettier_check.sh passes.
• Every behavioural statement in the new text was checked against main with temporary sqllogictest cases, rather than read from the code alone: the ignored LIMIT; the pre-statement values in SET a = b, b = a; the error text for an external table and for a view; the scalar
  subquery error; the IN and EXISTS all-rows result; and the EXPLAIN side effect. Those cases are not part of this PR, because the last two assert behaviour that should change.

Are there any user-facing changes?

Documentation only. No change to any API.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.36%. Comparing base (a6e2d3f) to head (8dfa9c1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
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- Coverage   81.36%   81.36%   -0.01%     
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  Files        1117     1117              
  Lines      397872   397916      +44     
  Branches   397872   397916      +44     
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+ Hits       323725   323751      +26     
- Misses      55229    55243      +14     
- Partials    18918    18922       +4     

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