update settings doc

Author: dschuff

site/source/docs/tools_reference/settings_reference.rst

@@ -2186,7 +2186,8 @@ WASM_BIGINT
 
 WebAssembly integration with JavaScript BigInt. When enabled we don't need to
 legalize i64s into pairs of i32s, as the wasm VM will use a BigInt where an
-i64 is used.
+i64 is used. If WASM_BIGINT is present, the default minimum supported browser
+versions will be increased to the min version that supports BigInt.
 
 Default value: true