docs: fix parameter syntax inconsistency for MySQL and SQLite

[kotahorii]

Summary

This PR fixes parameter syntax inconsistency in the howto documentation where examples only showed PostgreSQL syntax ($1, $2), causing confusion and errors for MySQL and SQLite users.

Problem

As reported in #3697, the generic howto documentation files showed only PostgreSQL parameter syntax:

• Used $1, $2 placeholders which only work with PostgreSQL
• Caused silent failures in MySQL code generation (functions generated without parameters)
• Caused syntax errors in SQLite compilation
• Created confusion for developers using multi-database setups

Solution

Updated four key documentation files to include database-specific examples:

Modified Files:

• docs/howto/update.md
• docs/howto/insert.md
• docs/howto/delete.md
• docs/howto/select.md

Changes Made:

✅ Added database-specific sections with clear syntax examples:

• PostgreSQL: $1, $2 syntax
• MySQL and SQLite: ? placeholder syntax

✅ Added explanatory notes about parameter binding differences

✅ Maintained existing Go code examples (they remain database-agnostic)

✅ Added clarification about MySQL not supporting RETURNING clause

Example of Changes

Before:

-- name: UpdateAuthor :exec
UPDATE authors SET bio = $2 WHERE id = $1;

After:

**PostgreSQL:**
-- name: UpdateAuthor :exec
UPDATE authors SET bio = $2 WHERE id = $1;

**MySQL and SQLite:**
-- name: UpdateAuthor :exec
UPDATE authors SET bio = ? WHERE id = ?;

Impact

• ✅ Eliminates confusion for MySQL/SQLite users
• ✅ Prevents silent code generation failures
• ✅ Provides clear guidance for multi-database projects
• ✅ Maintains backward compatibility (PostgreSQL examples preserved)
• ✅ Aligns with existing database-specific tutorial documentation

Test Plan

• Verified all syntax examples are database-appropriate
• Confirmed tutorials already use correct database-specific syntax
• Ensured documentation structure remains consistent
• Cross-referenced with official database documentation

Fixes #3697

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