[andygrove]

Which issue does this PR close?

Part of #6550.

Rationale for this change

The 1.2.0 change log and the known-differences section of the 1.2.0 upgrade guide are on branch-1.2 (#7290, #7291 and #7287). main needs them too: the change log so that the site lists 1.2.0, and the upgrade guide because the guide on main is the one that carries forward to later releases.

What changes are included in this PR?

Two commits.

• Change log. Adds docs/source/changelog/1.2.0.md as it stands on branch-1.2 at 4bda15db3e plus docs: [branch-1.2] Add changelog for 1.2.0 #7290 and docs: [branch-1.2] add #7281 to the 1.2.0 changelog #7291, and lists it first in docs/source/changelog/index.md. If more changes land on branch-1.2 before the release candidate and the change log is regenerated there, this file needs the same update.
• Upgrade guide. Adds "Known Differences From Spark That Are New in 1.2.0" to the "Upgrading to Comet 1.2.0" section of docs/source/user-guide/latest/migration-guide.md, copied from docs: [branch-1.2] document known differences new in 1.2.0 in the upgrade guide #7287. It covers collect_set over floats beside count(DISTINCT) diverges from Spark on signed zeros and NaN #7203, RANGE window frame boundary overflow diverges from Spark for DATE and DECIMAL ORDER BY #5022 and Native string and binary min/max under-report memory in hash aggregates #6936.

How are these changes tested?

Prettier 3.9.10 passes on the three files. The text is copied from branch-1.2 unchanged.

[sunchao]

Reviewed ee93398bd2ec613e7dceb6b6bb5cbe2e62c97a98 across five independent scopes. No actionable findings. The changelog and added known-differences section match branch-1.2 at 7462460492e60a40970bb689e654843b74e17d8a; the documented scopes and configuration workarounds agree with the implementation and linked issues.

Current-head preflight and Required Checks are green. This changes documentation only; I did not regenerate the historical changelog inventory.

[andygrove]

Thanks @sunchao