docs: Added CARGO_BUILD_DIR_LAYOUT_V2 to unstable docs

Author: ranger-ross

src/doc/src/reference/unstable.md

@@ -2039,6 +2039,9 @@ enabled = true
 Enables the new build-dir filesystem layout.
 This layout change unblocks work towards caching and locking improvements.
 
+In addition to `-Zbuild-dir-new-layout`, `CARGO_BUILD_DIR_LAYOUT_V2` also exists as a way to opt in and out of the new layout during the transition period.
+`CARGO_BUILD_DIR_LAYOUT_V2=true` allows users to opt in to the new layout, even on stable.
+`CARGO_BUILD_DIR_LAYOUT_V2=false` allows users to opt out to the new layout. This includes post stablization but it's important to note that this is a temporary flag and will eventually be removed.
 
 ## compile-time-deps