docs: document DELETE and UPDATE in the SQL reference

[michaelsembwever]

Which issue does this PR close?

Documentation follow-up for #19142. Split from #24567 at the reviewer's request.

Rationale for this change

The SQL reference does not document DELETE or UPDATE. SQL users need their syntax, result format, and table support.

Add syntax, examples, table support, and concise links to known limitations. Split from #24567

What changes are included in this PR?

• Add syntax and examples for DELETE and UPDATE.
• Explain affected-row counts, NULL predicates, and assignment semantics.
• Summarize table support and link to known limitations.

Provider implementation documentation remains in #24567.

What is the testing strategy for this PR?

cargo fmt --all, the documentation formatter.

Are there any user-facing changes?

SQL reference documentation only.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.41%. Comparing base (923d64f) to head (308c20d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25553      +/-   ##
==========================================
- Coverage   82.41%   82.41%   -0.01%     
==========================================
  Files        1138     1138              
  Lines      435320   435320              
  Branches   435320   435320              
==========================================
- Hits       358772   358766       -6     
- Misses      54841    54846       +5     
- Partials    21707    21708       +1     

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