Correct locale support in size optimization docs

[dajiaohuang]

Fixes #4922.

Move locale disabling from the level 1 description to level 2, matching the default FMT_USE_LOCALE definition in base.h and the existing macro documentation. No runtime behavior changes.

Validation:

• MSVC compile-time probes for FMT_OPTIMIZE_SIZE=0/1/2 confirm default FMT_USE_LOCALE=1/1/0.
• Built the patched MkDocs site with the pinned documentation requirements, Doxygen 1.18.0 and PYTHONUTF8=1; checked the rendered configuration table. One existing chrono-format-specifications anchor warning remains. Without Python UTF-8 mode, the existing C++ handler cannot read Doxygen's UTF-8 XML under Windows cp1252.
• git diff --check passes.

[vitaut]

lint failures is unrelated (will go away on rebase)

[dajiaohuang]

Rebased onto upstream main at d90c336 (Apply cmake-format) in 7d00593. The failing cmake-format job was caused by the older merge base's CMakeLists.txt formatting; the PR remains a doc-only change. git diff --check origin/main...HEAD passes, and CI should rerun on the rebased head.

[vitaut]

Thank you