docs: fix spread order in defineRelationsPart Rule 1 example

[rivaldorose]

Closes #629

Problem

In the Rule 1 warning for defineRelationsPart, the collapsed "Why it's important?" callout compares two spread orders — but both sentences say the same thing:

Having { ...relations, ...part } will result in […]

and having { ...relations, ...part } will result in […]

The second block shows the outcome of the opposite order — posts: {}, with the comment "posts relations information will be lost" — because the empty placeholder from relations overwrites what part defined. As written, the reader can't see which order causes that.

The code sample directly above already labels the orders correctly, so only the prose is wrong.

Change

Second sentence becomes { ...part, ...relations }. One line per dialect:

• src/content/docs/pg/relations.mdx
• src/content/docs/mysql/relations.mdx
• src/content/docs/sqlite/relations.mdx

The issue mentions one page; the same paragraph is duplicated across these three, so all three are updated. cockroach, mssql and singlestore don't contain this passage.

Verification

• pnpm test — 29/29 passing
• pnpm build (astro check && astro build) — clean, 769 pages
• Checked the built HTML for all three dialects: each now renders one { ...relations, ...part } and one { ...part, ...relations }, and the "correct order" sentence above is unchanged