Update comments / documentation.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/file-system.cc

@@ -688,6 +688,7 @@ void moveFile(const Path & oldName, const Path & newName)
 bool isExecutableFileAmbient(const fs::path & exe) {
     // Check file type, because directory being executable means
     // something completely different.
+    // `is_regular_file` follows symlinks before checking.
     return std::filesystem::is_regular_file(exe)
         && access(exe.string().c_str(),
 #ifdef WIN32

src/libutil/file-system.hh

@@ -265,7 +265,7 @@ Path defaultTempDir();
 
 /**
  * Interpret `exe` as a location in the ambient file system and return
- * whether it exists AND is executable.
+ * whether it resolves to a file that is executable.
  */
 bool isExecutableFileAmbient(const std::filesystem::path & exe);