Document RVV_OFF environment variable in rvv-off comments

Author: EliahKagan

Cargo-zng.toml

@@ -43,8 +43,9 @@ cmake = "0.1.50"
 # targets, and currently only has an effect when `cmake` is used. It builds
 # binaries portable to machines without RVV, even if the build machine is
 # riscv64 and detected to support RVV. This is useful for building a more
-# portable binary, or on systems where auto-detection is incorrect
-# (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
+# portable binary, or on systems where auto-detection is incorrect (as in
+# https://github.com/zlib-ng/zlib-ng/issues/1705). Building with the `RVV_OFF`
+# environment variable set (to any value) is equivalent to this feature.
 rvv-off = []
 
 [lints.rust]

Cargo.toml

@@ -93,8 +93,9 @@ stock-zlib = []
 # targets, and currently only has an effect when `cmake` is used. It builds
 # binaries portable to machines without RVV, even if the build machine is
 # riscv64 and detected to support RVV. This is useful for building a more
-# portable binary, or on systems where auto-detection is incorrect
-# (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
+# portable binary, or on systems where auto-detection is incorrect (as in
+# https://github.com/zlib-ng/zlib-ng/issues/1705). Building with the `RVV_OFF`
+# environment variable set (to any value) is equivalent to this feature.
 rvv-off = []
 # Deprecated: the assembly routines are outdated, and either reduce performance
 # or cause segfaults.