Theme developer handbook: Improve documentation about template part areas.

[carolinan]

Issue Description

From https://github.com/WordPress/gutenberg/issues/49407

Because the visual label for the uncategorized template part area is "General", it is easy mix up the template part area name when listing template parts in theme.json. Improve the documentation so that it is clearer that the value for this template part area is uncategorized, not general.

Section of Page with the issue

https://developer.wordpress.org/themes/advanced-topics/theme-json/#assigning-template-parts

[github-actions[bot]]

Heads up @kafleg @carolinan @TeBenachi - the "themes" label was applied to this issue.

[zzap]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.