Say why it occasionally makes sense to force RVV off

This is adapted from the comment on the feature that was added
under the design that was previously explored. This way, the
description will be present somewhere, even though it may be less
discoverable here in `cmake.rs` than it would be in manifests.

zng/cmake.rs

@@ -26,6 +26,9 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
         {
             Some("OFF" | "NO" | "FALSE" | "0") => {
                 // Force RVV off. This turns off RVV entirely, as well as the runtime check for it.
+                // This is not usually necessary, but can be useful for building binaries portable
+                // to systems that do not support RVV but where auto-detection fails to identify
+                // this (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
                 cmake.define("WITH_RVV", "OFF");
             }
             Some("ON" | "YES" | "TRUE" | "1") => {