Document 'Description' info file key for Layouts

[NormPlum]

The documentation for Layouts info files (https://docs.backdropcms.org/documentation/creating-layout-templates) doesn't mention the description key even though it's used in core layouts.

Suggested wording and example:

description = Includes a header, top, three columns, bottom, and footer.

"A short description of the layout template. This description is displayed on the layout templates page at "Administration > Structure > Layouts > Layout templates"."

[bugfolder]

I've made this change at https://docs.backdropcms.org/documentation/creating-layout-templates. I'll leave this issue open for a while for any further comment or suggestions.