Clarify comment about Cow variant preservation for efficiency

Co-authored-by: Byron <[email protected]>

Author: Copilot

gix-config-value/src/path.rs

@@ -113,7 +113,9 @@ impl<'a> From<Cow<'a, BStr>> for Path<'a> {
     fn from(value: Cow<'a, BStr>) -> Self {
         // Check if the value starts with ":(optional)" prefix
         if value.starts_with(OPTIONAL_PREFIX) {
-            // Strip the prefix while preserving the Cow variant to avoid unnecessary allocations
+            // Strip the prefix while preserving the Cow variant for efficiency:
+            // - Borrowed data remains borrowed (no allocation)
+            // - Owned data is modified in-place using drain (no extra allocation)
             let stripped = match value {
                 Cow::Borrowed(b) => Cow::Borrowed(&b[OPTIONAL_PREFIX.len()..]),
                 Cow::Owned(mut b) => {