doc: document Temporal configure flags in BUILDING.md

Note the default-on status of Temporal support in Node.js 26 and
document the --v8-enable-temporal-support and
--v8-disable-temporal-support options along with the cargo and rustc
auto-detection behavior introduced in #61806.

Refs: #63225
Signed-off-by: ChrisJr404 <chris@hacknow.com>

Author: ChrisJr404

BUILDING.md

@@ -1045,11 +1045,24 @@ enable FIPS support in Node.js.
 Node.js supports the [Temporal](https://github.com/tc39/proposal-temporal) APIs, when
 linking statically or dynamically with a version of [temporal\_rs](https://github.com/boa-dev/temporal).
 
-To build Node.js with Temporal support, a Rust toolchain is required:
+Temporal support is enabled by default starting in Node.js 26. Building it
+requires a Rust toolchain:
 
 * rustc >= 1.82 (with LLVM >= 19)
 * cargo >= 1.82
 
+If `--v8-enable-temporal-support` and `--v8-disable-temporal-support` are both
+omitted, `configure.py` probes for `cargo` and `rustc`. If either is missing,
+a warning is printed and Temporal support is disabled.
+
+* Pass `--v8-enable-temporal-support` to `configure.py` to require Temporal
+  support. The build will stop with an error if `cargo` or `rustc` cannot be
+  found.
+* Pass `--v8-disable-temporal-support` to opt out of Temporal support and
+  remove the Rust toolchain requirement.
+
+Passing both options to `configure.py` is an error.
+
 ## Building Node.js with external core modules
 
 It is possible to specify one or more JavaScript text files to be bundled in