Add a note about the cost of `VLOG` in non-debug builds.

PiperOrigin-RevId: 617186944
Change-Id: Id540fb42b02f24ad92b8f3898231f01ac004e191

absl/log/log.h

@@ -233,6 +233,11 @@
 //
 //
 // See vlog_is_on.h for further documentation, including the usage of the
 // See vlog_is_on.h for further documentation, including the usage of the
 // --vmodule flag to log at different levels in different source files.
 // --vmodule flag to log at different levels in different source files.
+//
+// `VLOG` does not produce any output when verbose logging is not enabled.
+// However, simply testing whether verbose logging is enabled can be expensive.
+// If you don't intend to enable verbose logging in non-debug builds, consider
+// using `DVLOG` instead.
 #define VLOG(severity) ABSL_LOG_INTERNAL_VLOG_IMPL(severity)
 #define VLOG(severity) ABSL_LOG_INTERNAL_VLOG_IMPL(severity)
 
 
 // `DVLOG` behaves like `VLOG` in debug mode (i.e. `#ifndef NDEBUG`).
 // `DVLOG` behaves like `VLOG` in debug mode (i.e. `#ifndef NDEBUG`).