runtime/debug: BuildSetting does not document DefaultGODEBUG [1.24 backport]

[gopherbot]

@prattmic requested issue #66465 to be considered for backport to the next 1.24 minor release.

@gopherbot Please backport to 1.23 and 1.24. This is a documentation fix that makes working with compatibility GODEBUGs more clear.

[gopherbot]

Change https://go.dev/cl/671995 mentions this issue: [release-branch.go1.24] runtime/debug: document DefaultGODEBUG as a BuildSetting

[gopherbot]

Closed by merging CL 671995 (commit 664cf83) to release-branch.go1.24.